UNIVERSITY  OF  CALIFORNIA 
AT  LOS  ANGELES 


THE  ANATOMY  OF  WOODY  PLANTS 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


THE  BAKER  fc  TAYLOR  COMPANY 

NEW  YORK 

THE  CAMBRIDGE  UNIVERSITY  PRESS 

LONDON 

THE  MARUZEN-KABUSHIKI-KAISHA 

TOKYO,  OSAKA,  KYOTO,  FUKUOKA,  SENDAI 

THE  COMMERCIAL  PRESS,  LIMITED 
SHANGHAI 


Diagrammatic  figure  of  the  French  artichoke,  Cynara  Scolymus,  showing  distri- 
bution of  oil  canals  in  the  various  organs  and  regions.  For  explanation  see 
chapter  xxxi 


THE  ANATOMY  OF 
WOODY  PLANTS 

By 
EDWARD  CHARLES  JEFFREY 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


COPYRIGHT   1917   BY  EDWARD  CHARLES   JEFFREY 

ALL    RIGHTS    RESERVED.     PUBLISHED    OCTOBER    1917 

Fourth  Impression  December  /pjo 


GO,  ILLINOIS,  U.S.A. 


PREFACE 

It  is  now  forty  years  since  De  Bary's  classic  Comparative 
Anatomy  of  the  Vegetative  Organs  of  the  Phanerogams  and  Ferns 
made  its  appearance.  In  the  interval  much  has  been  added  to 
our  knowledge,  particularly  in  the  paleobotanical  and  experimental 
fields.  The  doctrine  of  descent,  too,  has  now  reached  a  degree  of 
prominence  and  importance  which  it  did  not  possess  in  De  Bary's 
time.  As  a  consequence,  it  is  desirable  that  the  general  subject 
of  the  anatomy  of  the  woody  or  so-called  vascular  plants  should 
be  reviewed,  with  special  reference  to  its  historical  and  experi- 
mental  aspects.  This  is  perhaps  all  the  more  desirable  as  an 
effective  counterpoise  to  the  extreme  mechanistic  tendencies  of 
the  time.  It  will  accordingly  serve  a  useful  purpose  to  indicate 
how  large  a  part  of  the  organization  of  existing  plants  is  an  inherit- 
ance from  their  ancestors  of  earlier  geological  times. 

In  De  Bary's  textbook  both  paleobotany  and  development 
are  deliberately  eschewed.  The  first  of  these  is  now  essential  for 
any  adequate  comprehension  of  comparative  anatomy  in  its 
all-important  evolutionary  aspects.  It  is  abundantly  clear  that 
the  most  fruitful  results  from  the  standpoint  of  the  doctrine  of 
descent  are  to  be  derived  from  the  comparative  study  of  extinct 
and  existing  plants  belonging  to  the  same  orders,  families,  or 
genera.  It  is,  moreover,  obvious  that  the  living  forms  cannot 
be  interpreted  without  a  knowledge  of  their  past,  and  that  to 
an  even  greater  degree  the  organization  of  fossil  plants  is  a  closed 
book  to  those  who  are  unfamiliar  with  the  anatomy  of  allied  and 
still  living  types.  The  wide  range  of  facts  which  must  of  necessity 
be  covered  calls  for  a  somewhat  brief  and  even  elementary  treat- 
ment. Fortunately,  since  De  Bary's  time,  it  has  become  more 
and  more  evident  that  the  study  of  the  development  of  organs 
and  tissues  throws  little  trustworthy  light  on  the  processes  of 
evolution,  and  consequently  that  aspect  of  our  subject  need 
receive  no  more  attention  than  was  vouchsafed  to  it  by  the  great 
German  anatomist  nearly  half  a  century  ago. 


163199 


vi  PREFACE 

In  the  seventeenth  chapter  are  summarized  the  important 
general  principles  derived  from  the  investigation  of  related  living 
and  extinct  organisms.  The  beginning  of  the  studies  leading 
to  the  formulation  of  these  anatomical  canons  stands  largely  to 
the  credit  of  French  and  English  paleobotanists.  Since  they 
have  worked  mainly  with  Paleozoic  types,  their  activities  have 
been  preponderantly  in  the  direction  of  comparisons  between  the 
organization  of  the  earlier  cryptogams  and  gymnosperms  and 
their  still  living  survivors.  It  has  been  in  some  measure  the 
good  fortune  of  American  anatomists  to  continue  the  lines  of 
investigation  thus  begun  and  to  extend  them  to  the  study  of 
Mesozoic  and  still  living  gymnosperms.  The  extremely  harmoni- 
ous conclusions  resulting  from  the  anatomical  comparison  of  both 
Paleozoic  and  Mesozoic  forms  with  their  surviving  descendants 
have  justified  the  extension  of  the  same  principles  to  the  evolu- 
tionary investigation  of  other  woody  plants  (particularly  to  the 
angiosperms),  concerning  the  geological  past  of  which  we  are  still 
ignorant.  The  canons  formulated  in  chapter  xvii  have  as  a  conse- 
quence been  regarded  of  such  importance  that  any  conclusions  not 
in  harmony  with  them  have  ordinarily  not  been  considered  in  the 
following  pages  unless  they  have  held  the  ground  for  many  years 
or  are  at  the  present  time  advocated  by  anatomists  of  great  emi- 
nence: On  account  of  the  large  field  covered  in  the  present  neces- 
sarily elementary  work,  this  procedure  has  been  regarded  as 
essential,  and  it  is  hoped  that,  with  the  explanation  offered,  it 
will  not  appear  to  the  reader  too  dogmatic. 

The  author  has  been  fortunate  in  utilizing  the  services  of  his 
students  in  the  preparation  and  illustration  of  this  volume.  He 
is  particularly  indebted  to  Mr.  R.  E.  Torrey  for  his  skilful  and 
artistic  execution  of  a  large  number  of  the  figures.  Help  in 
this  respect  has  also  been  supplied  by  Mr.  R.  C.  Staebner  and 
Mr.  Charles  Drechsler.  Miss  Ruth  Cole  has  rendered  invaluable 
aid  in  the  onerous  task  of  preparing  the  text  and  reading  the 
proof.  Miss  Edith  S.  Whitaker  has  also  assisted  in  the  prepa- 
ration of  the  index. 

To  Professor  M.  A.  Chrysler,  of  the  University  of  Maine,  the 
author  owes  illustrations  of  secondary  growth  in  monocotyledons, 


PREFACE  vii 

and  to  Miss  Eloise  Gerry,  of  the  United  States  Forest  Service, 
an  admirable  photograph  elucidating  the  "bars  of  Sanio"  in 
coniferous  wood.  Last  but  not  least,  the  author  records  the 
valuable  services  of  Mr.  James  Austin,  assistant  in  the  labora- 
tories, in  connection  with  the  preparation  of  the  numerous  photo- 
graphic illustrations.  Dr.  D.  H.  Scott  and  his  publishers,  Messrs. 
A.  and  C.  Black,  have  very  kindly  permitted  the  reproduction  of 
a  number  of  figures  from  the  admirable  Studies  in  Fossil  Botany, 
which  are  among  the  comparatively  few  illustrations  in  the  present 
work  which  are  not  original.  To  Dr.  R.  T.  Jackson  the  author 
is  indebted  for  the  opportunity  of  photographing  for  illustration 
a  number  of  the  sections  of  carboniferous  plants  under  his  care 
in  the  Botanical  Museum. 

Professor  John  M.  Coulter  has  been  good  enough  to  read  the 
proofs  as  the  present  volume  passed  through  the  press.  To  my 
friend  Mr.  R.  W.  Sayles,  Director  of  the  Geological  Museum,  I 
am  indebted  for  valuable  criticisms  in  the  field  of  paleoclimatology 
and  also  for  other  essential  aid.  The  author  is  alone  responsible 
for  the  views  expressed  and  for  any  errors. 

BOTANICAL  LABORATORIES,  HARVARD  UNIVERSITY 
June,  1917 


CONTENTS 

CHAPTER  PAGE 

I.  THE  CELL i 

II.  THE  TISSUE  SYSTEMS   . .  8 

III.  THE  FIBROVASCULAR  TISSUES:  WOOD — GENERAL       ...  14 

IV.  THE    FIBROVASCULAR   TISSUES:    SECONDARY    WOOD — TRA- 

CHEIDS  AND  FIBERS 24 

V.  THE  FIBROVASCULAR  TISSUES:   SECONDARY  WOOD — PAREN- 
CHYMA         37 

VI.  THE  FIBROVASCULAR  TISSUES:    SECONDARY  WOOD — RAYS  .  61 

VII.  THE  FIBROVASCULAR  TISSUES:   SECONDARY  WOOD— VESSELS  92 

VIII.  THE  FIBROVASCULAR  TISSUES:  PHLOEM 107 

IX.  THE  EPIDERMIS 126 

X.  THE  FUNDAMENTAL  TISSUES 132 

XI.  DEFINITIONS  OF  THE  ORGANS 136 

XII.  THE  ROOT 143 

XIII.  THE  STEM 162 

XIV.  THE  LEAF 199 

XV.  THE  MICROSPORANGIUM 214 

XVI.  THE  MEGASPORANGIUM  AND  SEED 223 

XVII.  THE  CANONS  OF  COMPARATIVE  ANATOMY     ......  234 

XVIII.  THE  LYCOPSDDA  AND  PTEROPSIDA 244 

XIX.  THE  LYCOPODIALFS 252 

XX.  THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES)      .     .     .  264 

XXI.  THE  FILICALES 277 

XXII.  THE  ARCHIGYMNOSPERMAE:    CYCADOFILICALES  AND  CYCA- 

DALES        292 

XXIII.  THE  ARCHIGYMNOSPERMAE:   CORDAITALES  AND  GINKGOALES  305 

XXIV.  THE  METAGYMNOSPERMAE:  CONIFERALES         317 

XXV.  THE  METAGYMNOSPERMAE:  GNETALES 357 

XXVI.  THE  ANGIOSPERMS 373 


x  CONTENTS 

CHAPTER  PACK 

XXVn.  THE  WOODY  DICOTYLEDONS 379 

XXVIII.  THE  HERBACEOUS  DICOTYLEDONS '387 

XXIX.  THE  MONOCOTYLEDONS •    .     .  409 

XXX.  ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION    .     .     .  417 

XXXI.  EVOLUTIONARY  PRINCIPLES  EXHIBITED  BY  THE  COMPOSITAE  433 

XXXII.  ANATOMICAL  TECHNIQUE 444 

INDEX 473 


CHAPTER  I 

THE  CELL 

It  has  been  recognized  since  the  seventeenth  century  that  living 
beings,  particularly  plants,  are  composed  of  cells.  The  English 
investigators  Hooke  and  Grew  in  the  latter  half  of  that  century 
noted  the  fact  that  the  body  of  vegetable  organisms  was  often  con- 
stituted of  minute  chambers,  and  to  these  Hooke  first  gave  the 
name  of  cells.  Grew  was  apparently  the  originator  of  the  term 
"tissue,"  and  he  compared  the  organization  of  plants  with  the 
woven  texture  of  lace.  A  clear  conception  of  the  cellular  structure 
of  living  beings  was  not,  however,  reached  until  nearly  two  centu- 
ries later.  Plants  differ  from  animals  in  the  fact  that  their  gross 
internal  organization  is  of  relatively  slight  scientific  importance 
compared  with  the  more  obvious  bony  and  muscular  structures  of 
the  animal  body.  The  anatomy  of  plants  is  thus  essentially  a 
matter  for  microscopic  investigation.  Some  knowledge  of  the 
general  features  of  the  structure  of  the  cell  in  plants  is  accordingly 
necessary  as  a  preliminary  to  the  more  detailed  pursuit  of  anatomy. 

The  cell  in  vascular  plants  has  certain  features  which  bring  it 
into  sharp  contrast  with  the  corresponding  unit  of  structure  in  the 
higher  animals.  The  essential  substance  of  all  living  cells  is  proto- 
plasm, simply  distinguished  from  inanimate  matter  by  the  posses- 
sion of  a  capacity  for  change  and  reproduction,  which  does  not 
characterize  matter  devoid  of  life.  In  the  higher  plants  the  proto- 
plasm does  not  show  the  large  degree  of  solidity  which  is  a  feature 
of  the  animal  cell,  but  is  ordinarily  reduced  to  a  thin  vesicle  sur- 
rounding a  larger  or  smaller  central  cavity  known  as  the  vacuole. 
The  bladder-like  protoplasmic  vesicle  is  rendered  possible  in  plants 
by  a  containing,  supporting,  and  likewise  more  resistant  envelope 
called  the  cell  wall.  This  wall  is  not  nitrogenous  in  its  chemical 
composition,  as  is  true  of  the  living  protoplasmic  body,  but  is,  primi- 
tively at  least,  a  ternary  compound,  containing  carbon,  oxygen, 
and  hydrogen,  the  first-named  element  being  the  most  abundant. 


THE  ANATOMY  OF  WOODY  PLANTS 


Fig.  i  will  make  clear  the  organization  of  cells  in  an  immature 
seed  pod  or  pericarp  of  an  iris.  Each  element  obviously  consists 
of  a  large  central  cavity  as  noted  above,  variously  known  as  hydro- 
plastid,  hydroleucite,  tonoplast,  and  vacuole.  The  last  designa- 
tion seems  best  for  descriptive  purposes.  Surrounding  the  median 
space,  which  in  life  contains  water  with  various  substances  in 

solution,  is  the  proto- 
plasmic utricle.  The 
nature  of  the  latter 
varies  in  different 
cells.  In  the  ele- 
ments to  the  left  side 
of  the  illustration  the 
protoplasm  appears 
as  a  somewhat 
minutely  granular 
substance,  while  in 
the  cells  to  the  right 
the  structural  organi- 
zation of  the  proto- 
plasm is  marked  by 
the  presence  of  oval 
bodies,  the  chloro- 
plastids,  which  in  life 
color  are  green.  A 
dark  body  of  larger 
size  is  to  be  seen 

somewhere  against  the  cell  wall  and  imbedded  in  the  protoplasm. 
This  is  the  nucleus,  one  of  the  most  important  organs  of  the  cell, 
which  appears  to  preside  over  all  its  changes  and  activities. 
Another  body,  generally  larger  and  lighter  than  the  nucleus,  is 
frequently  present  in  the  cells  under  discussion — the  oilplastid 
or  eleoplast.  This  is  conspicuously  spongy  in  its  structure.  We 
may  now  turn  with  advantage  to  the  organization  of  the  cell 
wall.  On  the  left  side  of  the  illustration  the  common  outer  wall 
is  limited  by  a  distinct  membrane,  the  cuticle,  which  is  chemically 
different  from  the  rest  of  the  wall  substance  and  is  viable  to 


FIG.  i. — Transverse  section  of  young  pericarp  of 
Iris  species,  showing  organization  of  cells  of  epidermal 
and  fundamental  tissues. 


THE  CELL 


gases  but  not  to  fluids.  Below  the  cuticle  lies  the  cell  wall 
proper,  in  this  case  composed  of  cellulose.  The  cuticle  and 
its  underlying  wall  are  continuous  except  in  the  region  of  certain 
apertures,  the  stomata,  one  of  which  is  represented  in  the  figure. 
From  the  stoma  there  passes  inward  an  air  space,  which  quickly 
divides  into  fine  canals  lying  in  the  angles  between  the  internal 
cells.  These  are  intercellular  spaces  and  are  a  practically  unfailing 
accompaniment  of  the  living  elements  in  vascular  plants. 

The  next  illustration  (Fig.  2) 
visualizes  the  conditions  in  a  harder 
tissue,  namely,  one  of  the  wood  rays 
of  a  dicotyledon.  Here  the  mass  of 
cells  present  individually  somewhat 
rounded  contours,  and  in  .the  result- 
ing angular  interstices  appear  the 
intercellular  spaces.  The  protoplasm 
in  the  cells  under  consideration  is 
much  denser  than  in  the  elements  of 
the  wall  of  the  ovary  figured  above, 
and  accordingly  the  nucleus  lies  near 
the  geometrical  center  of  the  cell,  as 
is  commonly  the  case  in  highly  proto- 
plasmic elements,  in  contrast  to  its 
peripheral  position  in  those  in  which  the  protoplasm  is  only 
a  bounding  utricle.  The  cell  wall  in  the  present  instance  is,  rela- 
tively to  the  size  of  the  cells,  much  thicker  than  in  the  first  figure. 
As  a  consequence  of  this  increased  thickness  of  the  wall,  special 
devices  are  necessary  for  the  purpose  of  permitting  interchanges 
between  the  cellular  elements  and  their  environment.  The  wall 
is  thin  in  certain  definite  regions  for  the  attainment  of  this  end,  and 
these  locally  thin  spots  are  known  as  pits.  It  is  clear  that  the  pit 
of  one  cell  always  coincides  with  a  corresponding  pit  in  a  neighbor- 
ing element.  Further,  it  is  obvious  that,  although  the  cells  are 
in  close  relation  to  one  another  by  means  of  their  pits,  these  are 
prevented  from  becoming  actual  holes  by  the  persistence  of  the 
middle  lamella  or  cement  substance  as  the  membrane  of  the  pits. 
Not  all  the  pits,  however,  meet  a  corresponding  depression.  It 


FIG.  2. — Tangential  section  of 
cells  of  a  ray  in  a  dicotyledon 
(Drimys  species),  showing  pitting 
in  relation  to  cells  and  intercellu- 
lar spaces. 


THE  ANATOMY  OF  WOODY  PLANTS 


will  be  observed  that  there  are  well-developed  pits  toward  the  inter- 
cellular aerating  spaces.  These  are  known  as  air  pits  and  have 
no  counterpart  on  the  side  of  the  air  space.  They  are  commonly 
found  wherever  the  thickening  of  the  cell  wall  makes  difficult  the 
interchanges  between  the  living  protoplasm  and  the  outside  air. 
In  the  figure  under  consideration  the  cells  are  united  by  the  cement 
substance  of  the  middle  lamella,  which  appears  dark  and  contains 

the  air  spaces  within  its  sub- 
stance. Most  of  the  cells 
have  had  both  the  top  and 
the  bottom  walls  removed 
by  the  plane  of  section,  but 
in  one  or  two  the  cell  wall 
meets  the  eye.  Here  the 
pits  are  seen  in  face  view, 
and  it  becomes  clear  that 
they  vary  somewhat  in  size 
and  are  characteristically 
outlined  by  a  single  contour. 
On  account  of  this  condi- 
tion, which  results  from  the 


FIG.  3.— Section  of  inner  pericarp 
("stone")  of  a  peach,  showing  cells  with  ex- 
tremely thick  and  layered  walls  as  well  as 
numerous  elongated  pits. 


fact  that  the  bounding  walls 
do  not  overhang,  pits  of  this 
type  are  called  simple  pits. 
Communicating  pores  of  this  kind  are  found  in  cells  which  in 
their  functional  condition  contain  living  protoplasm. 

Let  us  now  turn  to  the  examination  of  cells  in  which  the  wall 
is  so  greatly  thickened  that  the  space  containing  the  protoplasmic 
body  becomes  much  reduced  in  size.  Fig.  3  reproduces  some  of 
the  elements  of  a  peach  stone  which  has  been  softened  by  a  pro- 
longed sojourn  in  hydrofluoric  acid.  The  protoplasmic  structures 
have  been  largely  consumed  in  the  thickening  of  the  cell  wall. 
The  latter  shows  well-marked  indications  of  layering,  a  feature 
often  present  when  much  thickening  has  taken  place.  The  layer- 
ing doubtless  has  some  numerical  relation  to  the  age  of  the  wall  in 
days.  The  pits  are  of  special  interest  in  this  instance.  Their 
correspondence  in  adjacent  cells  is  as  marked  as  in  the  former 


THE  CELL 


5 


illustration.  In  many  cases  two  or  more  near-lying  pits  of  the  same 
cellular  element  become  confluent  as  they  pass  inward,  and  open 
into  the  cavity  of  the  cell  by  a  common  aperture.  The  cement 


FIG.  4.— Wood  of  the  spruce  (Picea  canadensis) ,  showing  ray  cells  with  simple 
pits  and  longitudinal  water-conducting  elements  with  bordered  pits. 

substance  is  strongly  developed  here  and  the  thickened  walls  of 
the  cells  are  lignified.  The  pits,  in  spite  of  the  complications  noted 
above,  are  to  be  regarded  as  simple  pits. 


THE  ANATOMY  OF  WOODY  PLANTS 


The  examples  of  cells  hitherto  considered  have  been  those  which 
in  their  functional  condition  contain  living  protoplasm.  It  is  now 
necessary  to  refer  to  those  very  important  histological  elements 
of  vascular  plants,  which  in  the  mature  condition  normally  con- 
tain no  protoplasm  and  serve  to  conduct  water.  Elements  of 
this  nature  are  typically  much  elongated  and  are  provided  with  a 
different  sort  of  intercommunicating  pitting  than  that  found  in 
the  case  of  those  which  in  the  active  state  shelter  a  protoplasmic 
body.  In  the  accompanying  figure  (Fig.  4)  are  seen  a  number  of 
cells  of  this  sort  cemented  together  by  the  middle  lamella  (repre- 
sented as  a  black  framework  around 
the  cell  walls).  The  pits  which 
bring  about  intercommunication 
are  figured  as  occurring  on  two  of 
the  four  nearly  parallel  sides  of  the 
tracheids  or  water-conducting  cells. 
Their  openings  have  overhanging 
margins,  and  the  membrane,  the 
presence  of  which  precludes  the 
possibility  of  actual  openings,  is 
much  thickened  in  the  middle  to 
constitute  the  so-called  torus.  The 
presence  of  overhanging  margins  clearly  distinguishes  this  type 
of  pore  from  that  found  in  the  walls  of  living  cells.  The  cement 
substance  uniting  the  cells  with  one  another  is  in  general  lignified 
(that  is,  has  undergone  that  somewhat  complex  and  obscure 
modification  chemically  known  as  lignification)  like  the  cell  walls 
which  it  holds  together,  but  is  likewise  partially  in  the  pectic  or 
mucilaginous  condition,  a  state  which  causes  it  to  absorb  hema- 
toxylin  strongly.  In  the  region  of  the  pit  membranes  the  middle 
lamella  becomes  pectic  cellulose,  and  here  water  passes  through  the 
walls  much  more  readily  than  it  does  elsewhere.  On  either  side 
of  the  figure  is  seen  a  ray  of  the  wood,  the  cells  of  which  are  in 
relation  with  the  tracheids  by  means  of  pits.  It  is  clear  that  the 
pits  which  bring  about  intercommunication  are  bordered  on  the 
side  of  the  elongated  element  of  the  wood  (the  tracheid)  and  are 


FIG.  5. — Diagram  of  simple  and 
bordered  pits  in  face  and  profile  views. 
In  the  center  a  bordered  pit  from 
heartwood  is  shown. 


THE  CELL  7 

simple  on  the  side  of  the  medullary-ray  cells.     Pits  of  this  nature 
are  called  half-bordered. 

In  Fig.  5  simple  and  bordered  pits  are  represented  diagram- 
matically  side  by  side  and  from  both  profile  and  face  view.  Ob- 
viously the  simple  pits  in  face  view  are  single  in  contour,  while 
those  which  are  bordered  have  a  triple  concentric  outline,  the  outer- 
most circle  corresponding  to  the  boundary  of  the  broad  membrane 
of  the  pit,  the  innermost  to  the  narrow  mouth,  and  the  intermediate 
representing  the  outline  of  the  torus.  The  distinction  between 
simple  and  bordered  pits  is  an  extremely  important  one,  particularly 
in  the  lower  groups  of  vascular  plants.  In  higher  forms  the  dis- 
tinction is  of  less  value,  but  the  presence  of  bordered  pores  in  vas- 
cular elements  in  general  still  indicates  that  an  important  function 
of  the  element  so  provided  is  the  transport  of  water.  It  should  be 
further  noted  that  elements  with  bordered  pits  are  usually  without 
intercellular  spaces,  while  those  in  which  the  pits  are  of  the  simple 
type  normally  possess  such  aerating  cavities.  Exceptions  to  this 
statement  are  found  ordinarily  only  in  plants  which  have  become 
highly  specialized  in  connection  with  resistance  to  drought. 


CHAPTER  II 
THE  TISSUE  SYSTEMS 

The  cells  in  higher  plants  are  generally  grouped  into  well-marked 
systems  of  elements  which  are  known  as  the  tissues.  Tissues  are 
sometimes  denned  as  aggregations  of  cells  performing  a  similar 
function.  This  definition  is,  however,  open  to  some  objections,  as 
is  also  that  which  describes  a  tissue  as  a  mass  of  cells  of  similar 
origin.  Characterization  of  tissues,  either  by  their  functions  or 
by  their  mode  of  origin,  seems  less  desirable  than  a  definition  which 
makes  clear  that  the  aggregations  of  cells  have  a  common  organiza- 
tion. From  the  standpoint  of  evolution  it  is  the  structural  features 
of  the  tissues  which  are  of  the  greatest  significance.  In  works 
dealing  with  so-called  physiological  plant  anatomy  the  functions 
of  the  tissues  rather  than  their  peculiarities  of  structure  are 
naturally  most  emphasized.  From  the  point  of  view  of  the  doc- 
trine of  descent  functional  features  are  of  less  significance,  since 
it  is  precisely  these  which  are  the  most  readily  modified  and  as  a 
consequence  furnish  the  least  valuable  indications  of  the  course 
of  evolutionary  development  in  any  given  large  group.  Because 
the  present  work  deals  with  anatomy  from  the  outlook  of  evolution, 
structural  organization  in  the  case  of  the  tissues  stands  in  the  fore- 
ground, although,  of  course,  the  question  of  the  functioning  of  the 
various  tissue  systems  cannot  be  left  out  of  view. 

It  is  an  interesting  general  fact  that  the  boundaries  between  the 
tissue  systems  are  much  more  marked  in  the  plants  which  are 
geologically  older  and  lower  in  the  scale  of  evolution  than  they  are 
in  the  higher  seed  plants,  the  conifers,  and  the  angiosperms. 
Further,  the  more  conservative  organs  of  the  higher  forms — namely, 
the  root  and  leaf — exemplify  a  sharper  delimitation  of  the  tissue 
systems  than  does  that  most  progressive  of  all  plant  parts,  the  stem. 

For  the  purpose  of  the  present  work  the  tissues  of  plants  may 
be  divided  into  three  distinct  systems,  which  can  be  most  easily 
identified  by  reference  to  the  accompanying  figure  (Fig.  6)  of  a 


THE  TISSUE  SYSTEMS  9 

transverse  section  of  the  creeping  stem  of  Pteris  aquilina.  Through- 
out the  area  of  the  figure  are  scattered  oval  masses,  the  fibrovas- 
cular  bundles.  These  are  very  sharply  marked  off  from  the  rest 
of  the  tissues  by  a  boundary  appearing  as  a  dark  circumscribing 
line.  On  the  outside  of  the  stem  is  the  integumentary  system,  or 


FIG.  6. — Transverse  section  of  the  rootstock  of  Pteris  aquilina,  showing  three 
categories  of  tissue — namely,  epidermal  (external) ,  vascular,  and  fundamental. 

epidermal  tissue,  consisting  of  a  single  layer  of  cells.  The  epidermis 
is  characteristically  uniseriate  in  the  lower  forms,  and  only  in  some 
of  the  higher  vascular  plants  does  it  become  a  multiple  layer.  The 
remaining  structures  of  the  stem  of  the  bracken  fern  belong  to  the 
fundamental  system.  The  next  illustration  (Fig.  7)  shows  a  part 
of  the  stem  of  Pteris  more  highly  magnified,  so  that  the  details  of 


10  THE  ANATOMY  OF  WOODY  PLANTS 

structure  may  be  more  easily  discerned .  External  is  the  single  layer 
of  the  epidermis,  made  up  of  cells  with  thick,  heavily  pitted  walls. 
With  the  greater  magnification  employed,  the  cellular  organization 
becomes  very  obvious.  Underneath  the  epidermal  layer  is  situated 
the  fundamental  system,  in  turn  clearly  outlined  against  the 
fibrovascular  tissues  by  a  striking  boundary  composed  of  a  single 
series  of  cells  with  dark  tanniniferous  contents.  This  is  the  most 
internal  layer  of  the  fundamental  system  and  is  known  either  as 
the  endodermis  or  as  the  phloeoterma  (the  latter  term  in  its  ety- 
mology indicating  the  inner  layer  of  the  cortex  or  fundamental 
tissue).  The  fundamental  tissues  are  characterized  locally  by 


FIG.  7. — A  portion  of  a  transverse  section  of  the  rootstock  of  Pteris  aquilina 
more  highly  magnified,  showing  the  three  tissue  systems. 

certain  bands  of  dark-brown  skeletal  tissue,  which  are  very  char- 
acteristic of  the  lower  vascular  plants  and  subserve  to  a  large  extent 
the  mechanical  function  which  in  the  higher  plants  is  attended  to 
by  the  fibrovascular  system.  Those  regions  of  the  fundamental 
system  which  are  not  mechanical  in  their  nature  take  over  the 
function  of  storage  and  are  crowded  with  granules  of  starch  forming 
a  cordon  around  the  periphery  of  the  cells.  Centrally  the  elements 
of  storage  show  the  presence  of  a  dark-brown  substance  in  the 
region  of  the  vacuole.  This  is  tannin-like  in  its  nature  and  is 
commonly  present  in  the  fundamental  system  of  ferns.  The 
fibrovascular  structures  stand  out  sharply  from  the  rest  of  the 
tissues  and  are  obviously  much  more  complicated  in  their  organiza- 
tion than  are  those  previously  considered.  The  fibrovascular  aggre- 
gations of  cells  are  of  the  greatest  anatomical  importance,  both 
because  their  very  complexity  of  structure  supplies  many  valuable 


THE  TISSUE  SYSTEMS  n 

features  for  comparison  which  may  be  utilized  in  the  study  of 
evolution,  and  because  their  conservatism  makes  them  on  the  whole 
the  least  variable  of  the  elements  entering  into  the  composition  of 
the  higher  plants.  Likewise,  by  reason  of  their  resistance  to  decay, 
they  are  more  likely  to  be  preserved  as  fossils.  Beyond  emphasiz- 
ing the  complexity  of  the  fibrovascular  system,  it  need  not  be 
further  considered  at  the  present  time. 


FiG.  8. — Transverse  section  of  the  leaf  of  the  white  pine  (Pinus  Strobus),  showing 
the  three  tissue  systems  in  a  leaf. 

We  may  next  consider  the  tissue  arrangements  in  another  of 
the  plant  organs,  namely,  the  leaf.  In  the  figure  (Fig.  8)  is  repre- 
sented the  transverse  section  of  the  needle  of  Pinus  strobus.  Ex- 
ternally the  epidermis  forms  a  boundary  of  a  single  row  of  cells, 
continuous  except  where  interrupted  by  the  occurrence  of  stomatic 
openings.  Beneath  the  epidermis  lies  a  layer  which  is  ordinarily 
known  as  the  hypoderma  and  is  more  strongly  developed  in  fossil 


THE  ANATOMY  OF  WOODY  PLANTS 


than  in  living  pines.  The  fibrovascular  strand  is  sharply  bounded 
in  the  median  region  by  the  endodermis,  a  layer  circular  in  con- 
figuration and  well  developed  in  Pinus,  though  often  absent  in  the 
higher  conifers.  The  organization  of  the  fibrovascular  strand  need 
not  particularly  occupy  our  attention  at  this  stage,  as  it  will  be 
considered  in  detail  more  appropriately  in  the  sequel.  It  is  enough 
to  note  that  its  upper  or  woody  part  is  composed  of  empty  cells 
often  showing  bordered  pits — in  other  words,  of  tracheids.  These 

are  continuous  on  the 
flanks  of  the  strand  with 
short-pitted  elements,  the 
transfusion  cells,  which  are 
of  great  interest  from  the 
evolutionary  standpoint. 
The  cellular  complex  lying 
outside  the  endodermis 
and  within  the  epidermis 
is  the  mesophyll,  the  repre- 
sentative of  the  funda- 
mental tissues  in  the  leaf. 
The  cells  of  the  mesophyll 
are  infolded  in  a  manner 

FIG.  9.— Transverse  section  of  the  root  of  characteristic  of  most 
the  sarsaparilla  (Smilax),  showing  the  three  Hving  -^  jn  the  meso_ 
systems  of  tissues  in  root  organs.  ,  . .  . .  .  .  . 

phyll  he  also,  on  the  lower 

side  of  the  leaf,  conspicuous  secretory  spaces,  the  resin  canals. 
In  the  root  of  the  pine,  as  in  the  leaf,  the  same  sharp  distinction 
between  the  fibrovascular  structures  and  the  fundamental  tissues 
is  present.  In  the  stem  of  the  conifers  generally,  however,  the 
limit  between  the  tissues  belonging  to  the  central  conducting 
cylinder  (the  fibrovascular  system)  has  become  obsolete  and  can  be 
judged  to  have  been  formerly  present  only  on  theoretical  grounds. 
In  the  case  of  the  root  of  vascular  plants  in  general,  from  the 
lowest  to  the  highest,  the  limit  between  conducting  or  fibrovascular 
tissues  and  the  fundamental  system  is  usually  very  distinct  and  is 
one  of  the  features  which  so  clearly  and  universally  mark  the  root 
as  the  most  conservative  of  the  organs  of  plants.  Fig.  9  illustrates 


THE  TISSUE  SYSTEMS  13 

the  situation  in  this  respect  for  the  monocotyledons,  which  may 
on  strong  grounds  be  considered  as  the  highest  of  the  seed  plants. 
Externally  is  the  piliferous  layer,  from  which  the  root  hairs  are 
derived,  and  which  may  be  considered  in  a  general  way  as  the  equiva- 
lent of  the  epidermis  of  the  stem  and  the  leaf.  The  central  region 
is  occupied  by  the  fibrovascular  system,  sharply  limited  by  the 
endodermis,  composed  of  cells  ordinarily  thick-walled.  Between 
the  endodermal  limiting  membrane  and  the  piliferous  layer  lies 
the  cortex  of  the  root,  and  this  corresponds  to  the  fundamental 
category  of  tissues  in  the  case  of  the  stem  and  leaf. 

It  will  be  clear  from  the  account  given  above  that  there  are  three 
tissue  systems  in  plants  which  are  very  distinct  in  lower  forms  and 
in  the  less  changeable  and  more  conservative  parts.  Of  these  the 
epidermal  tissues  are  always  clearly  limited  both  toward  the  out- 
side and  also  in  relation  to  the  tissue  system  which  lies  inside. 
The  boundaries  dividing  the  fibrovascular  from  the  fundamental 
tissues  are  often  less  plainly  indicated,  and  in  the  case  of  the  higher 
groups  of  plants,  particularly  those  in  which  the  secondary  growth 
is  strongly  developed,  may  disappear  altogether;  in  such  cases  the 
limits  of  the  tissues  can  only  be  inferred  from  comparative  and 
developmental  anatomy. 


CHAPTER  III 
THE  FIBRO VASCULAR  TISSUES:   WOOD— GENERAL 

Since  the  fibrovascular  tissues  are  on  the  whole  the  most  impor- 
tant in  the  organization  of  the  higher  plants  both  from  the  evolu- 
tionary and  from  the  physiological  standpoint,  it  will  be  well  to 
begin  with  their  anatomy.  The  most  conspicuous  and  best- 
developed  portion  of  the  fibro vascular  system  in  land  plants  is 
the  wood.  The  aggregation  of  elements  assembled  under  this 
heading  affords  also  the  best  exemplification  of  the  process  of 
evolution  in  the  higher  plants  as  the  result  of  the  progressive 
development  of  the  principle  of  division  of  labor  and  the  gradual 
adaptations  of  plants  to  the  more  complicated  conditions  of  life 
obtaining  in  later  geological  times.  The  resistance  of  the  wood  to 
the  organisms  of  decay  is  greater  than  that  of  any  other  common 
plant  tissue  except  those  possessing  cutinized  or  suberized  cell  walls. 
We  have,  consequently,  in  the  woody  structures  past  and  present 
an  almost  perfect  biological  document,  carrying  back  the  history 
of  plants  in  relation  to  their  changing  conditions  of  environment 
into  remote  epochs  of  our  earth's  history. 

In  beginning  the  discussion  of  the  organization  of  wood  it  will  be 
well  to  direct  our  attention  in  the  first  instance  to  the  contrasts  in 
structure  presented  by  woods  of  ancient  and  modern  types.  The 
first  illustration  (Fig.  10)  shows  us  the  situation  in  the  oak.  The 
wood  is  conspicuously  marked  into  areas  by  boundaries  running 
at  right  angles.  Crossing  the  figure  from  top  to  bottom  are  rows 
of  large  openings  which  represent  the  vessels  or  water-conducting 
tubes  of  our  modern  forest  trees.  It  is  clear  that  these  are  large 
only  along  the  lines  which  mark  the  beginning  of  each  year's  growth. 
Farther  out  the  vessels  become  suddenly  of  much  smaller  caliber. 
Not  only  is  the  ligneous  structure  of  the  oak  transversely  banded 
by  reason  of  the  strikingly  larger  size  of  the  vessels  which  signalize 
the  spring  development,  but  also  by  an  equally  significant  change 
in  the  diameter  and  the  distribution  of  other  more  or  less  highly 


FIBROVASCULAR  TISSUES:  WOOD  15 

differentiated  elements,  to  be  described  in  a  later  chapter.  At 
right  angles  to  the  annual  zones  of  growth  and  crossing  these  are 
the  large  wood  rays.  These  are  extremely  conspicuous  structures, 
and  as  a  result  of  a  variation  in  the  rate  of  growth  due  to  their 
presence  they  bring  about  very  evident  depressions  on  the  faces 
of  the  annual  rings.  The  intervals  between  the  large  wood  rays 
are  occupied  by  more 
numerous  linear  stor- 
age bands,  which  are 
but  a  single  row  of 
cells  in  thickness  and 
are  known  as  uniseri- 
ate  rays. 

A  marked  con- 
trast to  the  wood  of 
the  oak  is  presented 
by  the  ligneous 
organization  of  the 
Paleozoic  gymno- 
sperm  Cordaites,  illus- 
trated in  Fig.  ii. 
Here  the  annual 

rings,  SO  clearly  pres-         FIG.  10. — Transverse  section  of  the  wood  of  the  red 

ent  in    the   oak    as   a    oak  (Quercus  rubra),  showing  annual  rings  and  highly 

i       r      j-rr  •        differentiated  structure  which  characterizes  the  organiza- 

result  of  a  differentia-   tion  of  the  woody  cylinder  in  modern  trees 
tion  in  the  size  and 

character  of  the  elements  corresponding  to  regularly  recurring 
annual  changes,  are  conspicuous  by  their  absence.  This  situation 
is  directly  correlated  with  the  more  equable  annual  cycle  of  remote 
geological  times.  We  find  illustrated  in  the  case  of  Cordaites  abso- 
lutely no  indication  of  seasonal  changes  in  temperature  or  varia- 
tions in  other  important  conditions  of  a  periodic  or  seasonal  nature. 
Not  only  is  the  wood  monotonously  the  same  as  one  passes  from 
the  inner  regions  to  the  exterior  layers,  but  it  likewise  shows  slight 
differentiation  in  the  direction  from  left  to  right.  Large  rays  of 
the  oak  type  are  quite  absent,  and  the  radial  storage  strands  are 
entirely  linear  in  their  nature. 


1 6 


THE  ANATOMY  OF  WOODY  PLANTS 


Having  noted  the  varieties  involved  in  wood  structure  in  correla- 
tion to  the  more  variable  conditions  of  environment  present  in 
modern  times,  we  may  now  with  advantage  direct  our  attention 
to  other  features  of  organization  which  characterize  the  evolution 
of  the  woody  cylinder  in  the  higher  plants.  Fig.  12  illustrates 
the  transverse  section  of  the  wood  of  a  lepidodendrid,  an  ancient 

tree  of  the  Paleozoic 
age.  It  is  clear  in 
this  case  that  the 
wood  has  a  circular 
outline  correspond- 
ing to  that  of  the 
stem  as  a  whole. 
Although  there  is  no 
indication  of  the  ex- 
istence of  annual 
increments  of 
growth,  the  wood  is 
obviously  divided 
into  a  central  mass, 
in  which  the  cells 
are  irregularly  dis- 
posed, surrounded 
by  a  zone  regularly 
seriate  and  marked 
by  the  presence  of 

wood  rays.  The  portion  of  the  wood  which  shows  no  linear  disposi- 
tion of  its  elements  is  known  as  the  primary  wood.  This  region  of 
the  wood  is  sometimes  designated  the  " cryptogamic "  or  "old" 
wood,  because  it  is  particularly  characteristic  of  the  organization  of 
vascular  cryptogams  and  of  the  older  groups  of  plants  generally. 
As  will  be  made  clear  later,  the  structure  and  mode  of  development 
of  wood  of  this  category  is  of  considerable  importance  from  the 
evolutionary  point  of  view.  The  zone  of  secondary  wood,  outside 
the  primary  or  cryptogamic  ligneous  core,  is  conspicuous  by  reason 
of  its  regular  radial  seriation  and  the  presence  of  storage  bands 


FIG.  ii. — Wood  of  a  Paleozoic  gymnosperm  from 
Prince  Edward  Island,  Canada,  showing  absence  of 
annual  rings  and  extremely  simple  organization. 


FIBROYASCULAR  TISSUES:   WOOD 


called  wood  rays.  The  secondary  wood  need  not  further  occupy 
us  in  the  present  chapter. 

Turning  our  attention  now  to  the  longitudinal  organization  of 
the  primary  wood,  we  find  it  characterized  by  the  presence  of  certain 
elements  appearing  in  a  somewhat  regular  sequence.  The  general 
situation  is  represented  in  Fig.  13.  On  the  left  of  the  diagrammatic 
illustration  lies  an  elon- 
gated thin-walled  cell 
marked  by  the  presence 
of  spiral  strengthening 
bands.  In  its  present 
state  this  element  is  de- 
void of  protoplasmic  sub- 
stance, but  in  an  earlier 
phase,  as  indicated  in  the 
next  figure  (Fig.  14),  liv- 
ing matter  was  present 
and  specially  aggregated 
in  the  regions  of  the 
thickened  spiral  bands. 
To  the  right  lies  a  second 
element  in  which  the 
strengthening  horizontal  P3 

ridges,  which   reinforce  FlG  I2._Trans verse  section  of  a  stem  of  a 

from  the  inside   the  gen-       lepidodendrid   trunk   from   the   Paleozoic  (after 

erally  thin  walls    of     the       Scott)>  showing  the   strong  distinction  between 

j        .  ,,  secondary  (radially  seriate)  and  primary  (unseri- 

water-conducting  cell,  are      ate)  xylem  characteristic  of  ancient  forms. 
nearer  to  one  another  and 

in  some  instances  are  more  or  less  united.  By  accentuation  of  the 
condition  of  approximation,  fusion  between  the  bands  results  and 
we  have  as  a  consequence  the  presence  of  the  scalariform  or  reticu- 
late tracheid.  In  general,  among  the  Pteridophyta  this  is  the 
extreme  stage  of  evolution  of  the  tracheary  cells  of  the  primary 
wood,  but  occasionally  among  the  more  complicated  and  extinct 
vascular  cryptogams,  and  characteristically  in  all  seed  plants,  the 
final  state  of  the  primary  wood  is  characterized  by  the  presence  of 


1 8  THE  ANATOMY  OF  WOODY  PLANTS 

the  pitted  element,  marking  a  distinct  advance  on  the  scalariform 
and  reticulate  tracheids  of  the  primary  ligneous  organization  of 
the  ferns  and  their  allies.  In  the  case  of  the  primary  wood  those 
elements  which  have  in  their  walls  thickenings  of  the  nature  of 
rings  and  spirals  are  ordinarily  designated  the  protoxylem.  This 


FIG.  13. — Diagrammatic  longitudinal  section 
of  the  fibrovascular  tissues  of  a  dicotyledon  (after 
Sachs),  showing  organization  of  primary  wood. 

characterization  of  the  first-formed  portion  of 
the  primary  wood  is  not  always  justified  by  the 
structures  present,  because  in  slowly  growing 
organs  and  in  subterranean  parts  even  of  rapid 
development  typical  ringed  and  spiral  elements 
may  be  nearly  or  quite  absent.  Usually,  how- 
ever, the  protoxylem  as  defined  is  the  ligneous 
structure  present  when  the  organ  is  undergoing 
rapid  elongation  and  by  its  constitution  permits 
of  accommodation  by  stretching  to  correspond 
with  the  increase  in  length.  Its  elements  as  a 
consequence  are  frequently  drawn  out  and  almost 
obliterated.  The  cells  of  the  primary  wood 
which  are  thickened  in  the  reticulate,  scalari- 
form, or  pitted  manner  are  formed  after  elongation  has  ceased, 
since  by  their  organization  they  are  incapable  of  increasing  their 


A 

FIG.  14. — Dia- 
grammatic views 
of  a  young  ele- 
ment of  the  pri- 
mary wood.  In  A 
the  normal  condi- 
tion is  shown, 
while  in  B  the  pro- 
toplasm has  been 
caused  to  contract 
by  means  of  plas- 
molysis. 


FIBROVASCULAR  TISSUES:  WOOD 


length.  Elements  in  the  aggregation  belonging  to  this  category 
are  known  as  the  metaxylem  or,  more  specifically,  as  the  primary 
metaxylem. 

In  the  preceding  paragraph,  for  the  sake  of  convenience  it  has 
been  assumed  that  the  order  of  development  of  the  primary  wood 
is  always  in  the  same  direction.  As  a  matter  of  fact  the  time  and 
the  order  of  appear- 
ance of  the  elements 
in  this,  from  the 
evolutionary  stand- 
point, highly  signifi- 
cant tissue  vary 
within  certain  im- 
portant limits.  We 
may  first  consider 
the  most  ancient 
order  of  seriation  of 
the  constituents  of 
the  primary  wood — 
that  found  in  the 
stems  of  the  most 
antique  plants  and 
in  the  roots  of  all 
vascular  organisms 
from  the  lowest  to 
the  highest.  Fig.  1 5 
illustrates  the  organization  of  the  wood  in  a  stem  of  the  common 
club  moss,  Lycopodium.  The  tissues  of  the  xylem  constitute  a  sort 
of  star,  the  points  of  which  are  occupied  by  the  small-sized  elements 
of  the  protoxylem.  As  the  rays  of  the  star  broaden  inwardly,  there  is 
a  transition  from  protoxylem  to  metaxylem.  The  situation  becomes 
more  clear  by  reference  to  transverse  sections  of  the  root  in  a  fern 
shown  in  Fig.  i6a.  Spiral  sculpture  marks  the  small  elements  on 
the  outside,  while  toward  the  center  of  the  organ  the  typical  sculp- 
ture of  the  metaxylem  becomes  more  and  more  conspicuous.  In 
the  club  mosses  and  their  allies,  as  well  as  in  all  roots,  the  seriation 
in  the  development  of  the  elements  of  the  primary  wood  is  very 


FIG.  15. — Transverse  section  of  the  upright  stem  of 
Lycopodium  davatum,  showing  centripetal  or  centrad 
development  of  the  primary  wood;  the  smaller  elements 
represent  the  protoxylem. 


20  THE  ANATOMY  OF  WOODY  PLANTS 

generally  from  the  exterior  toward  the  center,  and  the  metaxylem, 
as  a  consequence,  is  more  axial  or  central  in  position  than  the 
protoxylem.  This  mode  of  development  of  the  primary  wood  is 
characteristic  of  the  most  ancient  plants,  the  lycopods  and  their 
allies,  and  is  likewise  universally  present  in  the  most  conservative 
organ  of  all  plants,  the  root.  When  the  primary  woody  tissues 
develop  from  the  outer  region  inward,  as  indicated  above,  they  are 
said  to  be  exarch.  The  situation  just  described  will  become  more 
apparent  by  reference  to  Fig.  i6b,  which  shows  i6a  in  an  immature 


FIG.  1 6.— a,  transverse  section  of  the  bundle  of  an  old  root  of  Osmunda  cinna- 
momea,  showing  primary  wood  complete;  b,  younger  condition  of  the  same  with 
central  region  (metaxylem)  of  bundle  still  immature  and  containing  protoplasm. 

condition.  The  outer  regions  of  the  oval  mass  of  xylem  are  alone 
developed,  the  center  being  still  occupied  with  thin  cells  filled  with 
protoplasm. 

In  the  stem  organs  of  the  ferns  and  lower  gymnosperms  a  some- 
what different  mode  of  development  of  the  primary  wood  is  char- 
acteristically present.  This  may  be  illustrated  by  reference  to  one 
of  the  fibrovascular  strands  of  the  bracken  fern,  Pteris  aquilina. 
The  smallest  elements  of  the  wood  are  situated  in  the  woody  tissue 
constituting  the  center  of  the  bundle.  As  in  the  case  of  Lycopodium 
and  its  allies,  the  smaller  first-formed  cells  belong  to  the  protoxylem. 
The  situation  which  presents  itself  in  the  later  development  of  the 
woody  strand  of  ferns,  however,  is  usually  quite  different  from  that 
found  in  the  lycopod  series.  In  the  ferns  the  tissues  of  the  primary 
metaxylem,  instead  of  lying  entirely  toward  the  center  of  the  organ 


FIBRO VASCULAR  TISSUES:   WOOD 


21 


in  the  exarch  condition  featured  in  the  lowest  vascular  plants, 
characteristically  surround  the  first-formed  ringed  and  spiral  ele- 
ments (the  protoxylem).  This  situation  so  frequently  presented 
by  the  ferns  and  lower  gymnosperms  is  designated  as  mesarch. 
The  longitudinal  topography  of  the  bundle  in  this  type  is  shown  in 
Fig.  17. 

Fig.  13,  described  at  the  outset,  pictures  the  relative  position  of 
the  constituents  of  the  primary  wood  in  the  stem  of  the  higher  seed 
plants,  the  Gnetales,  the  Con- 
iferales,  and  the  angiosperms. 
In  this  case,  since  the  seria- 
tion  of  the  successive  elements 
is  always  outward  from  an 
internal  starting-point,  the 
primary  wood  is  known  as 
endarch.  This  condition  is 
the  typical  one  for  all  the 
higher  plants,  and  no  form 
characterized  by  it  can,  with- 
out the  clearest  evidence,  be 
regarded  as  low  in  the  scale 

of  plants.  Although  the  endarch  condition  is  a  feature  of  the 
development  of  the  stem  and  generally  of  the  leaf  in  the  highest 
plants,  it  is  important  to  emphasize  at  this  stage  that  the  root, 
even  in  the  most  advanced  organisms,  betrays  its  extreme  con- 
servatism by  adhering  in  its  primary  structures  to  the  mode  of 
development  and  seriation  of  the  elements  characteristic  of  the 
most  ancient  known  plants,  the  lycopods  and  their  allies.  Fig.  18 
reveals  the  organization  of  a  young  root  of  the  American  larch. 
We  may  disregard  in  the  present  connection  all  but  the  central 
tissues  of  the  root.  Right  and  left  can  be  distinguished  two 
spaces,  the  resin  canals.  Inside  of  each  of  these  two  secretory 
cavities  lies  a  cluster  of  protoxylem,  distinguishable  by  the  small 
size  of  its  elements.  Toward  the  center  from  each  aggrega- 
tion of  protoxylem  extends  the  metaxylem,  which  in  the  young 
condition  represented  in  the  figure  has  not  yet  become  joined 
in  the  center. 


_ 


FIG.  17. — Longitudinal  section  of  a 
bundle  from  the  rootstock  of  Pteris  aquilina, 
showing  the  elements  of  the  protoxylem  in 
the  center  of  the  wood. 


22 


THE  ANATOMY  OF  WOODY  PLANTS 


It  may  be  summarily  stated  in  regard  to  the  present  chapter 
that  wood  in  the  older  types  is  much  simpler  in  structure  and  less 
differentiated  than  in  modern  forms.  Further,  in  most  cases  it  is 
necessary  to  distinguish  between  primary  and  secondary  wood. 


FIG.  18. — Transverse  section  of  the  young  root  of  the  American  larch.  The 
primary  wood  has  not  yet  been  developed  in  the  central  region  of  the  root.  The 
endodermis  separates  the  central  or  fibrovascular  region  of  the  root  from  the  external 
fundamental  tissue  or  cortex. 

The  former  is  first  to  appear  and  is  characterized  by  the  lack  of 
distinct  seriation  in  its  elements.  The  secondary  wood,  by  con- 
trast, is  formed  later  and  the  cells  which  constitute  its  structure 
are  arranged  in  rows  corresponding  to  radii;  files  of  storage  cells, 


FIBROVASCULAR  TISSUES:   WOOD  23 

known  as  wood  rays,  are  a  marked  feature  of  its  organization.  The 
primary  wood  consists  typically  of  two  regions:  one  composed  of 
elements  with  ringed  and  spiral  thickenings  and  capable  of  elonga- 
tion in  accordance  with  the  growth  of  the  organs;  the  other  con- 
stituted of  tracheary  cells  thickened  in  a  reticulate,  scalariform,  or 
pitted  fashion  and,  as  a  consequence,  incapable  of  extension  to 
meet  the  needs  of  lengthening  parts.  The  former  region  of  the 
primary  wood  is  designated  the  protoxylem  and  the  latter  the. 
metaxylem.  The  order  of  development  of  the  protoxylem  and 
metaxylem  differs  significantly  in  different  groups  of  plants.  In 
the  very  oldest  forms  the  progress  of  differentiation  is  entirely 
toward  the  center  (lycopods  and  their  allies).  In  the  ferns  and  the 
lower  gymnosperms  the  sequence  is  first  central  or  centripetal 
and  later  peripheral  or  centrifugal,  with  the  result  that  the  protoxy- 
lem occupies  a  median  position.  In  the  higher  gymnosperms  and 
in  the  angiosperms  the  inward  order  of  development  has  nearly  or 
quite  disappeared,  and,  as  a  consequence,  the  protoxylem  lies-  on 
the  inside  of  the  metaxylem,  which  is  formed  characteristically 
outward  or  centrifugally.  Technically  these  three  types  of  organi- 
zation of  the  primary  wood  are  designated  exarch,  mesarch,  and 
endarch.  Finally,  it  may  be  recalled  that  the  primary  wood  of 
the  root  of  all  vascular  plants  has  the  exarch  organization  of  the 
older  types  of  stems  and  exemplifies  the  fact  that  the  root  in  this 
respect  as  in  so  many  others  (to  be  shown  in  the  sequel)  is  the  most 
conservative  of  plant  organs. 


CHAPTER  IV 

THE  FIBROVASCULAR  TISSUES:    SECONDARY  WOOD— 
TRACHEIDS  AND  FIBERS 

Very  important  constituents  of  all  woody  organizations  are  the 
tracheids  and  fibers.  These  present  a  wide  range  of  structure 
from  the  lower  forms  to  the  higher  and  illustrate  some  interesting 
general  evolutionary  principles.  Fig.  19  is  a  highly  magnified 
transverse  view  of  the  wood  of  the  white  pine  (Pinus  strobus)  in  the 
region  of  transition  from  one  annual  ring  to  the  next.  Woods  of 
this  type  are  very  simple  and  consist 
mostly  of  elongated  tapering  elements 
with  bordered  pits  in  their  walls  and 
known  as  tracheids.  The  only  fea- 
tures of  organization  not  tracheary  in 
their  nature  are  the  rays  and  the 
resin  cavity.  The  tracheids  are  dis- 
tinctly of  two  kinds.  Some  are  large 
and  thinner-walled  and  begin  the 
annual  ring  as  the  so-called  spring 
elements.  Others,  thicker  as  to  their 
walls  and  with  a  smaller  lumen  or  cen- 
tral cavity,  constitute  the  summer 
tracheids.  The  two  kinds  of  tra- 
cheids are  in  further  contrast  with 
one  another  because  of  the  position 
of  the  bordered  pits  on  their  walls.  In  the  spring  elements  the 
pits  are  confined  to  the  radial  walls — that  is,  those  sides  of  the 
fibers  which  are  either  in  actual  contact  with,  or  are  parallel  to, 
rays.  In  the  case  of  the  summer  tracheary  cells  pits  are  pre- 
dominant on  the  tangential  walls  which  are  at  right  angles  to 
the  rays.  In  addition  to  being  hi  communication  with  one 
another  by  means  of  bordered  pits,  the  tracheids  both  of  the 
spring  and  of  the  summer  wood  are  likewise  related  to  the  rays  by 


FIG.  19.— Part  of  a  transverse 
section  of  the  wood  of  the  white 
pine  (Pinus  Strobus),  showing 
radial  and  tangential  pitting  of 
the  tracheids. 


FIBRO VASCULAR  TISSUES:  TRACHEIDS  AND  FIBERS 


pits  which  are  bordered  on  the  side  of  the  tracheids  and  are  simple 
on  the  side  of  the  elements  of  the  rays. 

In  order  to  form  an  adequate  conception  of  the  nature  of  the 
fibrous  or  tracheary  elements  of  coniferous  woods  it  is  necessary 
to  view  them  in  isolation  from  the  tissues  of  which  they  form  so 
essential  a  part.  It  will  be  advanta- 
geous to  consider  a  simpler  condition 
first  and  then  to  proceed  to  the  more 
complex  situation  presented  by  the 
elongated  elements  of  the  wood  of 
the  pine.  In  Fig.  20  are  shown  tra- 
cheids of  the  Big  Tree  (Sequoia 
gigantea)  belonging  to  the  spring  and 
summer  growth  respectively.  On  the 
left,  one  of  the  spring  tracheids  is 
seen  from  its  radial  face.  The  cell  is 
obviously  bluntly  tapering  at  the  ends 
and  has  a  length  many  times  that  of 
its  diameter.  The  pits  which  orna- 
ment the  radial  aspect  are  of  two 
sizes.  The  larger  bordered  pits  are 
those  which  connect  tracheid  with 
tracheid.  The  smaller  pores  bring 
about  relations  between  the  rays  and 
the  tracheids.  The  latter  are  bor- 
dered only  on  the  tracheary  side, 
although  this  feature  is  naturally  not 
obvious  in  the  illustration.  The  tra- 
cheid is  represented  as  still  surrounded 
by  its  cement  substance,  which  is  indi- 
cated by  a  heavier  line.  On  the 
lateral  or  tangential  walls  of  the  cell  may  be  seen  the  profile  aspect 
of  other  bordered  pits.  These  are  tangential  pits  and  are  very 
rarely  present  in  the  spring  wood  of  conifers.  Fig.  2ob  reproduces 
the  appearance  of  the  same  tracheid  from  the  tangential  side. 
The  general  configuration  of  the  fibrous  element  is  now  much  more 
pointed  and  only  a  few  pits  can  be  seen  in  face.  On  the  side 


© 
© 

u 

FIG.  20. — Tracheids  of  the 
Big  Tree  (Sequoia  gigantea) .  Ex- 
planation in  the  text. 


26 


THE  ANATOMY  OF  WOODY  PLANTS 


walls  in  this  position  numerous  pores  may  be  distinguished,  in 
profile,  forming  the  principal  means  of  communication  between 
tracheid  and  tracheid  and  the  sole  one  between  tracheid  and  ray. 
To  the  right  of  the  figure  are  to  be  seen  the  radial  c  and  tangential 
d  views  of  a  summer  tracheid.  The  narrower  lumen  and  thicker 
walls  are  conspicuous.  In  the  case 
of  the  pitting  the  most  marked  fea- 
ture of  contrast  with  the  spring  fibers 
is  the  greater  number  of  bordered 
pits  occurring  on  the  tangential  walls. 
The  situation  in  Pinus  may 
now  be  advantageously  considered. 
Figs.  210  and  b  reproduce  the  spring 
tracheary  elements  of  this  genus  from 
the  same  aspects  as  represented  in 
the  case  of  Sequoia.  Beginning  with 
the  spring  element  on  the  left,  it  is 
clear  that  the  pits  in  relation  to  the 
rays  show  a  considerable  degree  of 
differentiation,  since  they  consist  of 
two  categories — namely,  small,  dis- 
tinctly bordered  pores  which  form 
the  intermediary  between  marginal 
ray  cells  and  tracheids,  and  large, 
angular,  scarcely  bordered  apertures 
uniting  the  central  ray  cells  with  the 
tracheids.  The  more  pointed  tan- 
gential contour  of  the  tracheid  in  b 
surrounds  an  area  entirely  free  from 
pits,  a  situation  nearly  universal  for 
the  spring  elements  of  coniferous 
woods.  In  the  lateral  walls  of  b 
numerous  radial  pits  are  seen  in  profile.  On  the  right  of  the  figure 
are  shown  the  corresponding  views  of  the  summer  tracheids  in  Pinus. 
The  narrower  diameter  and  thicker  walls,  as  well  as  the  numerous 
tangential  pores,  clearly  differentiate  elements  terminating  the 
annual  growth  from  those  formed  at  the  beginning  of  the  year. 


CO 


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00] 


00\\ 

ol 


B 


FIG.  21. — Tracheids  of  the  pine. 
Explanation  in  the  text. 


FIBROVASCULAR  TISSUES:  TRACHEIDS  AND  FIBERS       27 

Tangential  pits  are  so  obviously  and  constantly  a  criterion  of 
structure  at  the  end  of  the  annual  rings  in  conifers  that  they  may 
be  used  for  the  purpose  of  distinguishing  the  yearly  increments  in 
tropical  species,  in  which,  by  reason  of  the  slightly  marked  seasonal 
conditions,  the  zonal  woody  bands  are  indistinctly  indicated  or  even 
apparently  absent.  This  is  notably  the  case,  for  example,  in  the 
tropical  or  subtropical  species  of  the  genus  Araucaria. 

The  phenomenon  of  annual  rings  is  very  closely  correlated  with 
the  appearance  of  tangential  pits,  and  the  general  phenomena  in 
this  respect  are  worthy  of  much  greater  attention  than  has  hitherto 
been  devoted  to  them.  The  incipient  yearly  zones  in  trunks  of 
trees  of  earlier  ages  are  the  clearest  indication  of  the  progressive 
climatic  refrigeration  of  the  earth,  and  these  become  ever  more 
marked  in  later  geological  times.  Secondly,  it  will  be  made  clear  in 
the  sequel  that  the  modification  of  the  annual  ring  in  response  to 
inclement  seasonal  conditions  has  been  on  the  whole  the  most 
important  factor  in  the  evolutionary  development  of  plants  from 
the  earlier  epochs  to  the  present.  A  consideration  of  the  organiza- 
tion of  the  wood  in  a  gymnosperm  without  annual  rings — that  is,  a 
seed  plant  of  Paleozoic  time — is  of  extreme  interest  in  the  present 
connection.  The  tracheids  of  the  secondary  wood  of  the  gymno- 
sperms  of  this  age,  without  exception  so  far  as  is  at  present  known, 
had  the  pitting  confined  to  the  radial  walls;  as  a  consequence  water 
could  move,  easily  at  any  rate,  only  in  a  spiral  and  tangentially 
through  the  tracheary  elements.  The  truth  of  this  situation  may 
readily  be  grasped  by  reference  to  Fig.  1 1,  in  chapter  iii,  representing 
wood  of  the  Paleozoic  gymnosperm  Cordaites.  Details  of  the 
structure  of  the  wood  in  this  genus  can  be  gathered  from  the 
inspection  of  Fig.  22  in  the  present  chapter.  It  is  clear  that 
the  tracheids  communicate  with  one  another  and  with  the  cells  of 
the  uniseriate  rays  by  radial  pits.  Pits  are  conspicuously  absent 
on  the  tangential  walls  of  all  the  fibrous  elements  of  the  wood.  It 
will  be  obvious  from  the  facts  put  forward  in  this  paragraph  that 
the  distinction  between  spring  and  summer  tracheids  did  not 
exist  in  the  case  of  Paleozoic  woods;  in  other  words,  the  modifica- 
tions in  structure  and  pitting  which  have  become  a  fixed  feature  of 
the  organization  of  the  summer  tracheids  of  trees  of  the  Mesozoic 


28  THE  ANATOMY  OF  WOODY  PLANTS 

and  Tertiary  had  not  yet  made  their  appearance  in  the  Paleozoic 
age.  It  has  been  suggested  that  the  function  of  the  tangential 
pits  in  the  summer  wood  is  the  rapid  supply  of  water  to  the 


FIG.  22. — Wood  of  Paleozoic  gymnosperm  Cordaites.    Explanation  in  the  text 


cambium,  or  zone  of  growth,  in  the  reawakening  of  life  in  the  trunk 
after  the  winter  period  of  rest.  It  is  admissible  also  to  interpret 
the  greater  mechanical  efficiency  which  marks  the  summer  wood  of 
trees  possessing  annual  rings  as  of  advantage  in  resisting  the 


FIBRO VASCULAR  TISSUES:  TRACHEIDS  AND  FIBERS       29 

winter  storms  of  later  geologic  times.  It  is  unlikely  that  the  trees 
of  the  Paleozoic  age  were  exposed  to  the  often  furious  air  currents 
which  characterize  the  present  age  of  extreme  physiographic  and 
climatic  differentiation  of  the  surface  of  our  earth. 

It  is  convenient  at  this  point  to  introduce  a  general  statement 
in  regard  to  the  organization  of  the  wood  of  Paleozoic  and  Meso- 
zoic  forms  so  far  as  the  tracheary  elements  of  the  secondary  wood 
are  concerned.  In  the  case  of  the  giant  club  mosses  or  lepi- 
dodendrids  of  the  earlier  forests,  the  secondary  growth  consisted 
entirely  of  tracheids  with  scalariform  markings  similar  to  those 
characteristic  of  the  primary  wood  of  many  forms.  These  elements 
were  provided  with  pits  on  both  their  radial  and  their  tangential 
walls,  in  this  respect  offering  a  distinct  contrast,  as  likewise  in  their 
sculpture,  to  the  gymnosperms  of  the  same  period,  which,  as  has 
been  indicated  above,  possessed  only  pitted  tracheids  in  their 
secondary  growth  and  had  the  pitting  strictly  confined  to  the 
radial  surfaces  of  the  elements.  In  the  arboreal  forms  included 
under  the  general  heading  of  sigillarians  the  scalariform  sculpture 
often  gave  place  to  the  pitted  condition,  but  the  pores,  as  in  the 
lepidodendrids,  were  both  radial  and  tangential  in  distribution. 
In  the  sphenophyllums  and  their  allies,  the  calamites,  the  sculpture 
was  transitional  from  scalariform  to  pitted  and  was  confined  to 
the  radial  aspect  of  the  tracheids.  In  the  true  gymnosperms  of 
earlier  ages,  including  the  Cycadofilicales  (Pteridospermae)  and 
their  allies,  as  well  as  that  group  of  forms  included  under  the  con- 
venient cognomen  of  Cordaitales,  the  sculpture  of  the  secondary 
elements  of  the  wood  was  pitted  (not  scalariform)  and  was  con- 
fined to  the  radial  aspects  of  the  cells.  It  is  thus  clear  that  in  the 
great  Paleozoic  age  the  water-conducting  elements  of  the  secondary 
woody  growth,  so  generally  prominent  in  both  vascular  cryptogams 
and  gymnosperms,  were  much  more  uniformly  organized  than  is  the 
case  with  plants  of  later  geologic  times. 

The  further  course  of  evolution  and  differentiation  in  tracheary 
or  fibrous  elements  of  the  secondary  wood  can  be  studied  to  greatest 
advantage  in  the  angiosperms,  which,  although  they  first  appeared 
in  the  later  Mesozoic,  did  not  become  the  predominant  feature 
of  the  flora  until  Tertiary  times.  We  may  conveniently  begin  with 


3o  THE  ANATOMY  OF  WOODY  PLANTS 

the  examination  of  the  fibrous  structures  of  the  wood  of  the  oak. 
Fi01.  23  gives  a  general  view  of  the  organization  of  the  wood  in  a 
red  oak  (Quercus  rubra).  Clearly  it  shows  a  much  higher  degree  of 
complication  than  is  found  in  the  ligneous  tissues  of  a  conifer  or 


FIG.  23. — Wood  of  the  red  oak.     Explanation  in  the  text 

forms  still  lower  in  the  scale.  The  wood  is  crossed  by  conspicuous 
bands  of  storage  tissue,  the  large  rays  so  characteristic  of  the 
organization  of  oak  wood.  In  the  squares  bounded  by  the  annual 
rings  and  conspicuous  rays  lie  the  remaining  structures.  Of  these 
the  vessels  are  most  outstanding  and  are  very  large  at  the  beginning 
of  the  annual  zones,  becoming  much  smaller  in  caliber  in  the 
later-formed  wood.  The  vessels  are  imbedded  in  radially  directed 


FIBROVASCULAR  TISSUES:  TRACHEIDS  AND  FIBERS       31 


bands  of  tracheids  in  intimate  relation  to  the  vascular  ele- 
ments which  they  surround.  Laterally  to  the  stripes  of 
tracheids  in  a  general  radial  direction  lie  other  denser  areas 
made  up  of  mechanical  elements  known  as  fiber-tracheids. 
There  is  scarcely  any  marked  boundary  between  the  fiber- 
tracheid  and  the  tracheid  proper,  although 
the  extreme  conditions  of  both  are  distinct. 
Fig.  24  represents  these  two  kinds  of  elements 
as  seen  in  longitudinal  aspect  in  a  maceration 
of  the  wood.  The  tracheid  is  much  shorter 
and  broader,  has  thinner  walls,  and  is  abun- 
dantly pitted,  both  radially  and  tangentially, 
with  pores  provided  with  oblique  mouths. 
In  the  case  of  the  fiber-tracheids  we  have  to 
do  with  elements  of  considerable  length  and 
narrow  lumen,  inclosed  by  rather  thick  walls. 
The  pits  are  much  more  scanty,  but  are 
equally  distributed,  as  in  the  case  of  tracheids 
proper,  on  both  radial  and  tangential  walls. 
A  marked  feature  of  the  pits  in  these  ele- 
ments is  the  extreme  length  of  their  oblique 
mouths,  which  much  exceeds  the  diameter 
of  the  pit  membrane.  This  situation  becomes 
the  rule  in  tracheary  elements  devoted  more  to 
the  mechanical  than  to  the  water-conducting 
function. 

In  the  mass  of  woods  of  the  dicotyledons 
the  occurrence  of  tracheids  proper  and  ex- 
tremely differentiated  mechanical  elements 
side  by  side  with  gradual  transitions  is  not 
seen.  Usually  the  elongated  elements  of  the 
wood  other  than  vessels  (to  be  discussed  in  a 
later  chapter)  are  of  a  more  or  less  definite 
mechanical  nature  and  have,  as  a  conse- 
quence, lost  more  or  less  completely  the  characteristics  of 
tracheids.  Fig.  25  illustrates  several  types  of  fiber- 
tracheids  and  similar  structures.  In  a  is  seen  a  tracheid 


FIG.  24.— Tra- 
cheids and  fiber- 
tracheids  of  the 
oak. 


32 


THE  ANATOMY  OF  WOODY  PLANTS 


s 


FIG.  25.— Types 
of  fibers.  Explana- 
tion in  the  text. 


of  Ephedra,  a  low  representative  of 
the  Gnetales.  The  pitting  is  on  all 
walls,  and  spiral  bands  are  present 
on  the  inner  surface.  In  b  is  shown 
a  fiber-tracheid  from  a  species  of 
Magnolia.  Here  again  the  pitting  is 
not  confined  to  the  radial  aspects  of 
the  element,  but  is  also  tangential. 
The  apertures  of  the  pits  are  much 
elongated  and  extend  beyond  the 
nearly  circular  outline  of  the  pit 
membrane.  In  c  appears  a  type  of 
mechanical  element  characteristic  of 
the  higher  dicotyledons.  In  this  form 
of  fiber  the  pit  membrane  is  exceed- 
ingly narrow  and  the  mouth  extremely 
elongated.  As  a  result  of  this  situa- 
tion the  pores  of  the  fiber  appear 
to  be  practically  without  a  border. 
Mechanical  elements  of  this  type  are 
usually  known  as  libriform  fibers. 
It  has  been  considered  by  Strasburger 
and  others  that  the  libriform  mechani- 
cal element  is  of  a  different  morpho- 
logical nature  from  the  fiber-tracheid 
and  the  tracheid.  The  distinguished 
German  morphologist  was  of  the 
opinion  that  elements  of  this  type 
were  derived  from  the  fusion  of  stor- 
age parenchyma  cells.  This  view, 
however,  as  will  be  pointed  out  more 
appropriately  later,  does  not  har- 
monize with  the  general  evolutionary 
sequence  in  the  development  of 
structures  in  the  wood  and,  more- 
over, meets  with  serious  difficulties 
even  from  the  comparative  stand- 


FIBROVASCULAR  TISSUES:  TRACHEIDS  AND  FIBERS       33 


point.  For  example,  in  the  northern  species  which  are  placed 
under  the  genus  Fagus  the  mechanical  elements  are  of  the  nature 
of  fiber-tracheids  and  possess  clearly  bordered  pits.  In  the 
antarctic  species,  on  the  other  hand,  the  mechanical  structures  of 
the  wood  are  libriform  fibers.  We  have  thus  the  remarkable  situa- 
tion that  the  fibrous  elements  in  certain  species  of  the  same  genus 
are  derived  from  tracheids, 
while  in  others  they  are 
the  result  of  the  fusion  and 
modification  of  storage 
parenchyma  elements  in 
the  wood.  The  situation 
has  only  to  be  stated  to 
make  the  absurdity  of  the 
interpretation  obvious. 
Moreover,  while  in  the 
woods  of  the  Leguminosae 
the  mechanical  elements 


are,  for  the  most  part,  of 
the  nature   of    libriform 


FIG.  26. — Mucilaginous  fibers  of  the  black 
locust  (Robinia  Pseiidacacia). 

fibers,  in  Bocoa  provacensis 

the  mechanical  elements  are  represented  exceptionally  by  fiber- 
tracheids  with  clearly  bordered  pits.  In  certain  Rosaceae  the 
transition  from  fiber-tracheid  to  libriform  fiber  may  take  place 
within  the  limits  of  species  (Rhodotypus  kerrioides).  The  inter- 
pretation of  the  libriform  fiber  most  in  accord  at  once  with  the 
facts  of  comparative  anatomy  and  with  the  general  trend  of 
evolution,  as  inferred  from  the  comparison  of  existing  with  extinct 
types,  is  that  it  is  the  product  of  the  extreme  modification  of  the 
fiber-tracheid,  just  as  the  latter  in  turn  takes  its  origin  from  the 
less  specialized  tracheid. 

An  interesting  category  of  mechanical  fibrous  elements  is  sup-  1 
plied  by  the  mucilaginous  fibers  found  in  many  woods  of  widely 
separated  systematic  affinities.  In  these  elements  the  inner  portion! 
of  the  wall  has  become  more  or  less  completely  modified  into  al 
mucilaginous  state  which  causes  it  to  stain  strongly  with  hema- 
toxylin.  Fibers  of  this  type  are  found  commonly  in  certain 


34 


THE  ANATOMY  OF  WOODY  PLANTS 

and  are  widespread   in   leguminous  woods.^x 
The  presence  of  fibers  of  this  description  is 
often  of  value  from  the  hygroscopic  quality 
imparted  to  the  wood,  which  prevents  undue 
shrinking  or  swelling.     Those  species  of  oak 
with  mucilaginous  fibers  are  stated  to  be  of 
special  utility  for  cabinet  work  on  account  of 
their  relative  immunity  from  swelling  and 
shrinking.     In  the  case  of  the  black  locust 
(Robinia  Pseudacacid)  the  numer- 
ous mucilaginous  fibers  made  the 
wood  particularly  valuable  for  tree- 
nails in  the  days  of  the  construc- 
tion of  wooden  ships.     Obviously  a 
very  important  property  of  such  a 
wooden  spike  is  that  it  shall  neither 
swell  unduly  in  water  nor  shrink 
extremely  in  the  sun. 

Another  important  group  of 
fibers  is  that  included  under  the 
head  of  septate  fibers.  These  occur 
very  commonly  in  dicotyledonous 
woods,  particularly  in  shrubby  and 
herbaceous  forms  and  vines  belong- 
ing to  widely  separated  natural 
orders.  In  this  type  the  mechani- 
cal element  is  divided  transversely 
by  more  or  less  delicate  partitions 
of  a  cellulosic  or  pectic  nature, 
which  on  staining  with  hematoxylin 
and  safranin  stand  out  sharply 
from  the  lignified  longitudinal  walls 


B 


FIG.  27 


FIG.  28 


FIG.  27. — Septate  fibers  from  the  vine 
and  teak  (Tectona  grandis). 

FIG.  28.— Substitute  fiber  of  the  bar- 
berry. 


FIBRO VASCULAR  TISSUES:   TRACHEIDS  AND  FIBERS      35 

of  the  elements  in  question.  The  condition  here  present  is  repre- 
sented in  Fig.  27.  To  the  left  (a)  is  shown  a  septate  fiber 
from  the  grapevine,  while  on  the  right  appears  a  similar  element 
from  teak  (Tectona  grandis).  Elements  of  this  nature  are  the 
probable  basis  of  Strasburger's  apparently  badly  founded  view  that 
parenchyma  cells  by  their  fusion  have  given  rise  to  libriform  fibers. 
The  pits  in  septate  fibers  are  usually  much  larger  than  in  the  libri- 
form elements  and  are  conspicuously  simple. 

A  last  interesting  category  of  fibrous  elements,  also  characteristic 
of  extreme  types,  such  as  herbs,  shrubs,  and  vines,  is  the  so-called 
substitute  fiber  (Ersatzfaser  of  German  authors).  In  elements  of 
this  nature  septation  is  absent,  but  protoplasmic  contents  are 
present  in  the  cell  when  it  is  fully  developed  and  in  a  functional 
condition.  Fig.  28  shows  such  an  element  from  the  wood  of  the 
barberry.  The  pits  are  both  radial  and  tangential,  although  the 
former  are  obscured  in  illustration  by  the  presence  of  protoplasm. 
The  mechanical  elements  of  nearly  all  dicotyledonous  herbs  belong 
in  this  category.  The  substitute  fiber  represents  a  convenient 
union  of  the  functions  of  strength  and  storage  of  food  in  the  same 
element,  a  condition  particularly  advantageous  in  the  slender  stems 
of  vines  and  herbs. 

The  final  paragraph  of  this  chapter  may  appropriately  be 
devoted  to  the  subject  of  the  distribution  of  the  pitting  in  the  fibers 
of  the  dicotyledons.  It  has  been  noted  that  in  the  conifers  in 
general  the  pitting  of  the  tracheary  or  fibrous  elements  is  typically 
confined  to  the  radial  walls,  except  in  the  case  of  the  cells  termi- 
nating the  year's  growth  (summer  tracheids).  The  mechanical 
elements  of  the  end  of  the  annual  ring  have  tangential  as  well  as 
radial  pits.  In  the  case  of  the  Paleozoic  gymnosperms,  since 
there  are  (with  certain  unimportant  exceptions)  no  annual  rings 
marking  seasonal  diversity,  tangential  pitting  is  entirely  absent,  and 
the  pores  of  the  tracheids  are,  as  a  consequence,  exclusively  radial. 
The  dicotyledons  and  the  Gnetales  present  a  very  different  situation 
from  that  found  in  the  seed  plants  characteristic  of  Mesozoic  and 
Paleozoic  times;  for  in  the  former  the  mechanical  elements, 
whether  of  the  nature  of  tracheids,  fiber-tracheids,  libriform  fibers, 


36  THE  ANATOMY  OF  WOODY  PLANTS 

substitute  fibers,  or  septate  fibers,  are  particularized  by  the  fact 
that  the  pits  or  pores  are  not  of  exclusively  radial  distribution  in 
any  part  of  the  wood,  but  likewise  occur  in  numbers  on  the  tan- 
gential aspects  of  the  elements.  This  situation  in  the  woods  of 
later  times  is  of  special  significance  when  considered  in  connection 
with  other  features  of  structure  to  be  noted  in  a  later  chapter. 


CHAPTER  V 


THE  FIBROVASCULAR  TISSUES:    SECONDARY  WOOD- 
PARENCHYMA 

Although  the  present  chapter  is  to  be  devoted  to  the  presence 
of  parenchyma  in  the  secondary  wood,  it  will  be  advantageous 
to  consider  first  the  presence  of  cells  belonging  to  this  category  in 
the  primary  xylem.  Parenchyma tous  elements,  as  has  been  made 
clear  in  an  earlier  chapter,  are  those  which  are  characterized  in 
their  functional  con- 
dition by  the  pres- 
ence of  protoplasm, 
by  simple  pitting  of 
their  walls,  .and  by 
their  generally  more 
or  less  isodiametric . 
dimensions.  Cells  of 
this  nature  form  a 
well-marked  feature 
of  primary  wood — 
that  is,  ligneous  tis- 
sues which  are  with- 
out rays  and  without 
regular  disposition  of 
their  elements.  The 
parenchyma  of  the 
ancient  arboreal  club 
mosses  of  the  Paleo- 
zoic, the  lepidodendrids,  is  of  special  interest  from  the  standpoint 
of  the  doctrine  of  descent,  for  here  the  mode  of  origin  of  the 
parenchymatous  elements  of  the  primary  wood  is  clearly  indicated. 
Fig.  29  shows  a  transverse  view  of  the  primary  woody  tissues  of  a 
species  of  Lepidodendron  from  a  "coal-ball"  of  the  Carboniferous 
of  Lancashire  in  England.  A  number  of  elements  of  large  size 
and  thick  walls  can  be  distinguished.  These  are  the  tracheids.  In 

37 


FIG.  29. — Origin  of  wood  parenchyma  in  the  pri- 
mary wood  of  a  lepidodendrid. 


THE  ANATOMY  OF  WOODY  PLANTS 


addition  there  can  be  made  out  other  small  cells  which  are  not  so 
markedly  thickened.  Finally,  the  tissues  include  cells  with  walls 
which  are  distinctly  thin— the  wood  parenchyma.  In  the  case  of 
the  cells  with  moderately  thickened  walls,  where  the  bottom  (or 
top)  is  included  in  the  plane  of  section,  scalariform  thickenings  can 
readily  be  distinguished.  The  thin-walled  elements,  when  the  lower 


FIG.  30. — Longitudinal  section  of  the  primary  wood  of  a  lepidodendrid. 
planation  in  the  text. 


Ex- 


or  upper  wall  is  exposed  in  the  preparation,  show  it  to  be  without 
scalariform  sculpture. 

A  highly  important  light  is  thrown  on  the  facts  recorded  at 
the  end  of  the  preceding  paragraph  by  the  longitudinal  section  of 
the  same  primary  wood  exhibited  in  Fig.  30.  Here  may  be  seen 
a  number  of  long  structures  with  thick  walls  and  scalariform 
sculpture.  These  are  tracheids  of  the  primary  wood.  Here  and 
there  may  be  distinguished  other  structures  of  similar  length, 


FIBRO VASCULAR  TISSUES:  PARENCHYMA  39 

which  instead  of  being  simple  are  septate.  In  some  instances  the 
short  segments  of  the  longer  cells  are  obviously  tracheary,  since 
they  have  scalariform  sculpture  on  their  lateral  and  terminal  walls. 
In  other  instances  the  septations  of  the  long  elements  are  quite 
thin-walled  and  without  any  scalariform  or  reticulate  thickenings. 
Cells  of  the  latter  type  are  parenchyma tous,  and  the  origin  of  such 
elements  in  the  case  of  the  primary  wood  of  Lepidodendron  is 
clearly  indicated.  It  is  obvious  that,  as  a  consequence  of  the 
subdivision  of  originally  long  cells,  tracheary  in  their  character, 
two  sorts  of  products  result — namely,  short  thin-walled  cells  (the 
wood  parenchyma),  and  equally  abbreviated  elements,  sisters  to 
these,  which  constitute  merely  segments  of  the  subdivided  tra- 
cheids.  It  thus  becomes  clear  that  the  parenchymatous  elements 
of  the  primary  wood  in  the  very  ancient  genus  Lepidodendron  are 
derived  from  the  subdivision  of  elements  which  were  primitively 
tracheids.  It  will  be  shown  in  subsequent  paragraphs  that  the 
origin  of  wood  parenchyma  in  the  secondary  xylem  is  likewise  due 
to  the  septation  of  elements  originally  of  the  nature  of  tracheids. 
In  the  present  connection  the  question  naturally  arises  as  to  the 
derivation  in  turn  of  the  tracheids.  The  following  up  of  this 
problem  would  lead  us  too  far  into  the  field  of  purely  speculative 
evolution.  This  much,  however,  may  be  stated,  that,  accepting 
the  thallose  liverworts  as  the  probable  starting-point  of  the  evolu- 
tion of  vascular  forms,  it  becomes  clear  that  the  mass  of  elaters 
(with  spirally  thickened  walls)  which  constitute  the  columella  in 
Pellia,  Aneura,  and  some  species  of  Anthoceros,  etc.,  obviously 
constitute  the  prototype  of  the  fibrovascular  bundle  of  the  ferns 
and  higher  forms.  If  this  hypothesis  is  to  be  admitted,  and  it  is 
the  one  assumed  by  the  greater  number  of  those  who  have  worked 
on  the  somewhat  meager  facts  which  throw  light  on  the  probable 
origin  of  the  Pteridophyta,  it  appears  clear  that  in  the  spirally 
thickened  elaters  we  have  the  originals  of  the  spiral  elements  of  the 
protoxylem.  It  follows  that  elaters  are  the  forerunners  of  the 
tracheids,  which  in  turn,  by  subdivision,  as  evidenced  by  the  lepido- 
dendrids,  have  given  rise  to  the  parenchymatous  elements  of  the 
primary  wood.  There  does  not  seem,  however,  to  be  any  clear 
indication  in  the  case  of  other  and  existing  lycopodineous  forms 


4o  THE  ANATOMY  OF  WOODY  PLANTS 

which  throws  light  on  this  important  subject.  In  the  stem  of  the 
remaining  vascular  plants  the  problem  has  likewise  become  obscured 
by  lapse  of  time  and  the  blotting  out  of  the  original  and  primitive 
conditions.  There  seems  to  be  no  evidence  in  regard  to  the  origin 
of  parenchymatous  elements  in  primary  wood  other  than  that 
in  the  stem  of  the  lepidodendrids,  described  above.  The  modern 
descendants  of  the  group  do  not  furnish  elucidation  of  the  origin 
of  parenchymatous  elements,  and  the  same  statement  appears  to 
hold  in  the  case  of  other  living  and  extinct  representatives  of  the 
Pteridophyta. 

The  secondary  wood  of  the  older  and  Paleozoic  groups  of  plants 
was,  so  far  as  our  present  knowledge  goes,  entirely  without  paren- 
chymatous or  storage  elements  (with  the  exception  of  the  so-called 
medullary  rays  to  be  dealt  with  in  the  next  chapter).  This  is 
true  not  only  of  the  arboreal  cryptogams  of  the  ancient  period,  the 
lepidodendrids,  the  sigillarias,  the  sphenophyllums,  and  calamites, 
but  also  of  those  lower  and  more  primitive  gymnosperms  included 
under  the  general  headings  of  Cycadofilicales  (Pteridospermae) 
and  Cordaitales.  Not  only  are  the  Paleozoic  vascular  plants  not 
known  to  manifest  the  presence  of  true  xyliary  parenchyma,  but 
the  same  statement  appears  to  hold  in  regard  to  the  vascular  plants 
of  the  early  part  of  the  Mesozoic — that  is,  the  Trias.  Parenchyma- 
tous elements  have  not  yet  been  seen  in  any  American  wood  of  the 
earlier  Mesozoic.  It  is  only  in  the  Jurassic  period  that  paren- 
chymatous cells  clearly  manifest  themselves  as  a  feature  of  the 
organization  of  the  secondary  wood,  and  at  this  time  also  the 
annual  ring  as  a  feature  of  organization  of  the  woody  cylinder 
first  becomes  well  marked.  There  is,  in  fact,  a  distinct  correlation 
between  the  appearance  of  true  parenchymatous  storage  elements 
and  the  phenomenon  of  annual  rings. 

Before  referring  to  true  parenchymatous  structures  in  secondary 
wood  it  will  be  well  to  make  mention  of  structures  which  may 
readily  be  mistaken  for  these.  In  certain  plants  even  of  the 
Paleozoic  age  the  rays  have  attached  to  their  margins  more  or 
less  elongated  cells  which  are  often  of  such  length  that  they  form 
longitudinal  commissures  between  adjacent  rays.  A  situation  of 
this  kind  is  found,  for  example,  in  the  calamites  and  sphenophyllums 


FIBROVASCULAR  TISSUES:  PARENCHYMA  41 

and  is  quite  commonly  present  in  the  genus  Ginkgo,  a  living  sur- 
vivor of  a  group  anciently  numerous  in  genera  and  species.  A 
parallel  state  appears  in  Pinus,  a  living  genus  which  is  represented 
by  nearly  a  hundred  species  in  the  Northern  Hemisphere,  but 
which  had  probably  three  or  four  times  as  many  species  in  the  later 
Mesozoic.  These  vertical  commissures  between  the  rays  must 
be  considered  in  the 
same  category  with 
rays  and  do  not 
represent  veritable 
wood  parenchyma. 

Having  made  it 
clear  that  all  paren- 
chymatous  elements 
in  the  wood  cannot 
be  regarded  as  true 
xylem  parenchyma, 
we  find  it  possible  to 
discuss  to  greater 
advantage  structures 
properly  included 
under  this  heading. 

The   first    clear   CVl-          FIG.  3r.— Transverse  section  of  the  wood  of  the  root 
dence   in    regard  to    in  Picea. 
the  presence  of  cells 

belonging  to  this  category  dates  from  the  Jurassic  of  Northern 
Europe  as  observed  by  Gothan  and  Holden.  There  is  good  reason 
to  believe  that  the  original  region  of  appearance  of  the  storage 
elements  of  the  secondary  wood  is  at  the  end  of  the  annual 
rings.  There  is,  in  fact,  no  good  evidence  that  true  parenchymatous 
elements  of  the  wood  occur  anywhere  in  forms  not  characterized 
by  the  presence  of  annual  rings.  The  conditions  accompanying  the 
evolution  of  the  storage  elements  of  the  xylem  can  best  be  studied 
in  the  lower  representatives  of  the  pine  family  or  Abietineae.  The 
genus  Pinus  itself,  now  known  to  be  extremely  ancient  in  its 
occurrence,  does  not  exhibit  the  elements  under  consideration  in  a 
typical  form.  It  is  in  the  case  of  Picea,  Larix,  and  Pseudotsuga 


THE  ANATOMY  OF  WOODY  PLANTS 


that  they  manifest  themselves  in  the  most  significant  manner. 
The  genus  Picea  will  perhaps  serve  best  to  illustrate  the  conditions 
involved  in  the  appearance  of  parenchymatous  elements  in  the 
secondary  wood.  Fig.  32  illustrates  the  structure  of  a  transverse 
view  of  the  wood  of  the  root  of  Picea  canadensis.  Parts  of  two 
annual  rings  are  represented.  The  summer  wood  is  easily  recog- 
nized by  the  thicker 
walls  of  the  tracheids 
and  the  tangential  pits. 
It  passes  abruptly  into 
the  spring  wood  of  the 
next  annual  increment, 
characterized  by  large- 
sized  thin-walled  ele- 
ments with  entirely 
radial  pitting.  The  cru- 
cial feature  of  the  figure 
is  presented  by  certain 
cells  portrayed  in  black 
at  the  end  of  the  annual 
ring,  or,  as  it  is  com- 
monly phrased,  on  the 
face  of  the  summer  wood. 
These  elements  are  parenchymatous  and  constitute  the  only 
representatives  of  this  category  of  storage  tissue  in  the  wood  of 
Picea.  Fig.  33  presents  a  longitudinal  aspect  of  the  face  of  the 
summer  wood  in  the  same  species.  In  order  that  a  considerable 
length  may  be  shown,  three  successive  segments  of  the  plane  of 
section  are  depicted.  In  the  median  segment  a  row  of  short  cells  is 
seen.  These  have  obviously  simple  pits  in  their  walls  in  relation 
both  to  one  another  and  to  adjacent  elements  in  the  wood  belonging 
to  a  different  category.  In  other  words,  the  cells  under  considera- 
tion are  typical  parenchymatous  elements  in  longitudinal  view. 
To  the  left  lies  another  and  lower  segment.  At  the  top  of  this  is 
the  continuation  of  one  of  the  parenchymatous  elements  of  the 
first-described  segment.  As  we  follow  the  series  of  short  cells 
downward  in  the  segment  to  the  left  the  character  of  the  elements 


FIG.  32. — Detail  of  transverse  section  of  the 
vood  of  the  root  in  Picea. 


FIBROVASCULAR  TISSUES:  PARENCHYMA 


43 


changes.  They  now  show  bordered  pitting  instead  of  pores  of  the 
simple  type.  The  right-hand  longitudinal  segment  in  the  figure 
shows  the  region  above  that  represented  in  the  central  segment. 
Here  the  continuation  of  the  parenchymatous  elements  of  the 
median  segment  is  below,  and  this  type  of  cell  is  again  merged  into  a 
series  of  short  tracheids  derived  from  the  segmentation  of  one  of 
the  normal  longitudinal  elements  of  the  wood. 


FIG.  33. — Longitudinal  section  of  the  wood  of  the  root  in  Picea  canadensis. 
Explanation  in  the  text. 

The  general  situation  in  regard  to  the  mode  of  origin  of  longi- 
tudinal storage  elements  or  wood  parenchyma  in  the  gymnosperms 
can  perhaps  be  more  readily  illustrated  by  means  of  a  diagram. 
Fig.  34  represents  a  ray,  ordinary  tracheids,  and  a  septate  or 
divided  tracheid  in  relation  to  one  another  in  the  longitudinal 
aspect.  The  ray  need  not  be  specially  considered,  as  the  subject 
of  the  origin  and  organization  of  rays  is  to  be  discussed  in  the 
following  chapter.  Turning  our  attention  to  the  normal  tracheids 
represented  in  the  diagram,  we  find  it  clear  that  these  are  char- 
acterized by  the  possession  of  pits  both  on  the  radial  and  on  the 


44 


THE  ANATOMY  OF  WOODY  PLANTS 


tangential  walls,  a  feature  commonly  found  in  the  case  of  fibrous 
elements  in  the  terminal  region  of  the  annual  ring  in  the  conifers  and 
allied  groups.  The  septate  tracheid  is 
the  most  striking  feature  of  the  diagram. 
In  this  element  numerous  transverse  walls 
interrupt  the  continuity  of  the  central 
lumen.  Certain  of  these  transverse 
partitions  are  characterized  by  the  pres- 
ence of  bordered  pits,  while  others  offer  to 
the  eye  pits  of  the  simple  type.  In  a 
third  condition  bordered  and  simple  pits 
confront  one  another  in  the  same  wall. 
Those  cells  which  communicate  with  the 
surrounding  elements  by  means  of  simple 
pits  are  typically  occupied  by  protoplas- 
mic contents,  while  the  elements  possess- 
ing pores  of  the  bordered  category  are 
invariably  without  living  substance.  It 
will  be  clear  to  the  reader  that,  if  the 
diagrammatic  representation  presented  in 
Fig.  34  is  correct,  the  parenchyma tous 
elements  of  the  wood  come  from  the  sub- 
division of  the  primordial  elements  which 
would  in  other  cases  and  under  different 
circumstances  give  rise  to  ordinary  tra- 
cheids.  At  an  early  stage  these  elements 
became  transversely  septate,  and  in  the 
segments  so  set  off  the  protoplasm  some- 
times persists  (when  a  typical  parenchy- 
matous  element  of  the  wood  is  the  result) ; 
at  other  times  it  disappears  with  the 
complete  differentiation  of  the  walls 
surrounding  it  (hi  the  case  of  so-called 
short  tracheids).  An  interesting  fact  in 
this  connection  is  the  occurrence  of  paren- 
chymatous  storage  elements  in  the  same 
region  of  the  wood  where  the  tangential 
pits  take  their  origin.  It  has  been  made 


FIG.  34. — Diagrammatic 
representation  of  the  origin 
of  wood  parenchyma  in  the 
Picea. 


FIBROVASCULAR  TISSUES:  PARENCHYMA  45 

clear  in  a  former  chapter  that  in  the  case  of  the  Paleozoic 
gymnosperms  there  are  at  the  same  time  no  annual  rings  (typically 
at  leas.t)  and  the  pitting  of  the  tracheary  elements  of  the  wood 
is  entirely  radial  in  its  disposition.  In  the  gymnosperms  of  the 
Mesozoic  age  both  annual  rings  and  tracheids  tangentially  pitted 
in  the  terminal  region  of  the  yearly  zones  of  growth  become  the 
rule.  It  is  highly  significant  in  this  connection  that  the  first 
appearance  of  true  longitudinal  storage  parenchyma  is  from  the 
Jurassic  (Middle  Mesozoic)  onward,  and  that  the  storage  cells  take 
their  origin  in  the  terminal  region  of  the  annual  rings  and  clearly 
show  from  their  mode  of  development  that  they  are  the  derivatives 
of  tracheary  elements,  since  all  stages  of  transition  between  septate 
tracheids  proper  and  files  of  parenchyma  are  found.  It  has  hjeen 
suggested  by  Strasburger  that  the  tangential  pitting  of  the  sum- 
mer tracheids  is  for  the  purpose  of  easily  and  rapidly  supplying 
water  to  the  cambium  in  the  next  opening  period  of  growth. 
Such  a  hypothesis  would  accord  well  with  the  very  definite  correla- 
tion between  tracheids  of  this  type  and  the  phenomenon  of  annual 
rings.  If  the  condition  of  annual  growth  be  accepted  as  the  prob- 
able elucidation  of  the  tangential  pitting  of  tracheids  in  more 
modern  gymnosperms,  it  is  not  difficult  to  put  forward  a  similar 
claim  for  the  terminal  and  tangential  parenchyma  which  probably 
represents  the  primitive  disposition  of  parenchymatous  elements  in 
gymnospermous  woods.  If  the  tangential  pitting  is  for  the  purpose 
of  supplying  the  awakening  cambium  with  water,  there  can  be  little 
doubt  that  the  later-appearing  and  correlated  feature  of  terminal 
longitudinal  storage  cells  is  significant  in  connection  with  the  need 
of  the  reviving  initial  cells  of  the  zone  of  growth  for  readily  avail- 
able food. 

In  Pinus,  as  indicated  above,  there  is  no  true  wood  parenchyma. 
In  the  allied  genera,  Picea,  Pseudotsuga,  and  Larix,  it  has  become 
clearly  and  unmistakably  established  and  occurs  everywhere  in  the 
wood  of  the  root,  even  when  it  is  absent  or  degenerate  in  the  stem. 
As  will  be  shown  later,  the  root  is  the  most  conservative  organ 
of  plants.  In  the  case  of  higher  members  of  the  pine  family  (Abie- 
tineae),  Cedrus,  Pseudolarix,  Abies,  and  Tsuga,  the  parenchymatous 
elements  are  still  found  typically  at  the  end  of  the  annual  ring,  but 
they  no  longer  normally  show  evidence  of  derivation  from  tracheids, 


46  THE  ANATOMY  OF  WOODY  PLANTS 

other  than  in  the  fact  that  they  are  found  in  spindle-shaped  or  fusi- 
form groups  resembling  in  contour  tracheary  elements.  Injured 
specimens  of  the  woods  of  the  four  genera  under  discussion  show 
(Fig.  35),  however,  the  transition  from  short  tracheids  to  true 
parenchyma  cells  as  the  result  of  the  septation  of  elements  laid 
down  by  the  cambium  as  tracheids.  This  is  an  example  of  the 


FIG.  35. — Injured  wood  of  Tsuga  canadensis,  showing  transition  of  tracheids 
to  parenchyma. 

recall  of  ancestral  conditions  as  a  consequence  of  injury,  a  phe- 
nomenon extremely  common  in  the  case  of  plants  with  secondary 
growth.  This  situation  is  classified,  as  will  be  shown  later,  under 
the  caption  of  the  evolutionary  doctrine  of  reversion. 

Already  in  the  higher  representatives  of  the  pinelike  conifers — 
namely,  Abies  and  Tsuga — the  parenchyma tous  elements  have 
not  only  lost  all  normal  indications  of  derivation  from  tracheary 
elements  at  the  end  of  the  zone  of  annual  growth,  but  have  even 
begun  to  abandon  the  strictly  terminal  position  in  the  yearly  rings 


FIBRO VASCULAR  TISSUES:  PARENCHYMA  47 

of  wood.  In  certain  species  of  Abies  and  Tsuga  the  location  of 
the  parenchyma  is  on  the  face  of  the  summer  wood  only,  while 
in  others  it  is  no  longer  strictly  confined  to  this  position.  In  the 
genus  Abies,  A.  webbiana  and  A.  cephalonica  are  characterized  by 
longitudinal  storage  cells  no  longer  wholly  terminal  in  position 
but  scattered  throughout  the  annual  growth.  The  same  condition 
is  present  in  Tsuga  mertensiana  and  in  smaller  branches  of  T. 
canadensis.  This  type  of  disposition  of  parenchyma  may  con- 
veniently be  designated  diffuse  to  distinguish  it  from  the  terminal 
condition  of  storage  elements  in  their  first  appearance  among  the 
conifers.  It  is  of  interest  to  note  that,  so  far  as  the  matter  has  been 
investigated,  both  Abies  and  Tsuga  in  the  root  have  terminal 
parenchyma  exclusively,  no  matter  what  the  situation  may  be  in  the 
case  of  the  stem.  The  diffusion  of  parenchymatous  cells  through- 
out the  annual  ring  in  higher  forms  of  seed  plants  has  its  significant 
analogy  in  the  spreading  of  tangential  pitting,  at  first  confined  to  the 
later-formed  tracheids  of  the  summer  wood,  to  the  elements  of  the 
rest  of  the  annual  ring  in  higher  types.  Diffuse  wood  parenchyma 
never  betrays  its  origin  by  normal  transitions  to  septate  tracheids. 
It  is  only  in  the  case  of  wounding  that  its  tracheary  origin  can  be 
distinctly  observed.  The  longitudinal  storage  cells,  to  which  the 
name  of  wood  parenchyma  is  given,  always  reveal  their  derivation 
from  the  fibrous  elements,  however,  by  the  fact  that  they  are 
grouped  in  series  corresponding  in  length  and  outline  to  the  pointed 
tracheids  constituting  primitively  the  sole  longitudinal  elements 
of  the  wood. 

In  the  case  of  the  abietineous  genera  Abies  and  Tsuga,  cited 
above,  it  is  quite  clear  that  the  storage  elements  of  the  woods  have  , 
in  some  instances  departed  from  the  exclusively  terminal  position 
and  have  become  distributed  throughout  the  annual  ring.  In  those 
conifers  belonging  to  the  subtribes  known  as  Taxodineae,  Cupres- 
sineae,  and  Podocarpineae  the  parenchyma  of  the  wood  is  char- 
acteristically diffuse  and  no  longer,  except  by  the  grouping  and 
contour  of  its  elements,  gives  evidence  of  tracheary  origin.  Injuries 
in  most  cases  reveal  conditions  of  transition  from  septate  tracheids 
to  pointed  rows  of  parenchymatous  elements.  In  the  subtribes 
Araucariineae  and  Taxineae  parenchyma  has  disappeared  from  the 


48 


THE  ANATOMY  OF  WOODY  PLANTS 


normal  structure  of  the  wood  in  the  vegetative  stem,  but  is  ordina- 
rily clearly  and  sometimes  even  abundantly  present  in  the  diffuse 
condition  in  regions  which  are  conservative  of  ancestral  characteris- 
tics, such  as  the  root,  cone  axis,  etc.  It  may,  moreover,  be  easily 
recalled  as  a  consequence  of  experimental  procedure.  It  is  accord- 
ingly clear  that,  from  the  evolutionary  standpoint  at  any  rate,  con- 
siderable care  is  necessary  in  deciding  as  to  the  primitive  presence 
or  absence  of  parenchymatous  elements  in  the  woody  tissues  of  the 
conifers. 


A  B  C 

FIG.  36. — Diffuse  parenchyma  in  a  coniferous  wood.     Explanation  in  the  text 

Fig.  36  shows  the  topography  of  the  diffuse  condition  of  paren- 
chyma in  coniferous  woods  in  three  dimensions.  In  a  is  seen  the 
transverse  view  of  a  wood  of  this  type.  The  longitudinal  storage 
elements,  or  wood  parenchyma,  are  represented  with  heavy  black 
walls  to  bring  out  by  contrast  their  distribution  in  the  general 
structure  and  at  the  same  time  the  frequent  chemical  difference 
of  then-  walls  from  the  tracheary  tissues  proper.  In  b  appears 
the  tangential  aspect  of  the  storage  elements,  in  which  they  present 
themselves  in  the  greatest  breadth  and  on  the  whole  show  most 
clearly  by  pits  their  relation  to  other  elements  of  the  wood.  In  the 
case  of  c  these  important  elements  of  wood  structure  are  revealed  in 
the  radial  aspect,  and  their  diffuse  disposition  is  as  apparent  as  in 
the  transverse  plane  of  section. 


FIBRO VASCULAR  TISSUES:   PARENCHYMA  49 

It  will  be  clear  from  the  account  of  the  appearance  of  paren- 
chymatous  cells  given  in  the  preceding  paragraphs  that  their  origin 
in  the  primary  tissues  of  the  wood  is  revealed  only  in  the  case  of  that 
very  ancient  group  of  cryptogamous  trees,  the  lepidodendrids  of 
the  Paleozoic  age,  where  they  are  very  clearly  derived  from  the 
septation  of  tracheids.  Further,  in  the  case  of  the  secondary  wood, 
parenchymatous  elements  properly  so  called  did  not  make  their 
appearance  before  the  Mesozoic  period  and  are  as  distinctly  corre- 
lated in  their  origin  with  the  phenomenon  of  annual  rings  pre- 
sented by  plants  of  the  Mesozoic  and  later  geologic  time  as  are 
tangential  pits  in  the  tracheids.  As  has  been  made  clear  in  an 
earlier  chapter,  the  secondary  wood  of  Paleozoic  gymnosperms  was 
characterized,  not  only  by  the  absence  of  annual  rings,  but  equally 
clearly  by  the  default  of  pits  on  the  tangential  walls  of  the  tracheids 
and  the  entire  absence  of  parenchymatous  elements  in  the  wood. 
The  arrival  of  the  phenomenon  of  annual  rings  can  be  most  satis- 
factorily explained  in  the  case  of  the  known  facts  by  the  hypothesis 
of  the  gradual  refrigeration  of  the  surface  of  our  earth,  with  the 
consequent  appearance  of  a  winter  period  of  rest  in  vegetative 
activity.  The  phenomenon  of  annual  growth  soon  brought  into 
prominence  the  two  striking  features  of  terminal  tangential  pitting 
and  terminal  storage  parenchyma.  Both  features  of  organization 
were  later  extended  with  greater  or  less  completeness  to  the  whole 
of  the  annual  ring.  It  is  further  quite  clear  that  parenchymatous 
storage  elements  came  into  being  in  the  secondary  wood  by  the 
septation  of  tracheids,  precisely  as  has  been  shown  to  be  the  case 
in  the  similar  elements  of  the  primary  wood  (apparently  only  clearly 
indicated  in  the  case  of  the  lepidodendroid  cryptogams  of  the 
Paleozoic  age). 

In  the  higher  gymnosperms,  including  the  greater  number  of 
conifers  as  well  as  the  Gnetales,  the  distribution  of  the  longi- 
tudinal storage  elements  of  the  secondary  wood  is  typically  and 
primitively  diffuse;  that  is,  such  cells  are  not  confined  to  the  end  of 
the  recurring  zones  of  growth,  but  are  scattered  throughout  the 
annual  ring.  It  is  apparently  not  necessary  to  figure  the  situation 
for  the  Gnetales,  since  it  corresponds  so  closely  with  that  found  in 


5° 


THE  ANATOMY  OF  WOODY  PLANTS 


the  higher  Coniferales.  Fig.  37  illustrates  the  distribution  of  the 
wood  parenchyma  in  those  dicotyledons  in  which  the  condition  is 
diffuse.  In  a  is  shown  a  transverse  view  of  the  secondary  wood  in 
the  black  walnut  (Juglans  nigra).  The  distribution  of  the  storage 
cells  is  made  clear  by  the  outlining  of  their  walls  in  solid  black. 
In  b  the  same  wood  is  depicted  in  longitudinal  radial  aspect,  and  the 
nature  and  distribution  of  the  parenchymatous  elements  become 
doubly  clear.  A  fact  not  without  interest  from  the  evolutionary 


FIG.  37. — Diffuse  parenchyma  in  Juglans.     Explanation  in  the  text 

standpoint  is  the  uniform  distribution  of  parenchyma  after  this 
fashion  in  whole  natural  orders  of  dicotyledons.  For  example,  in 
the  Juglandaceae,  Betulaceae,  Fagaceae,  Ebenaceae,  Ericaceae, 
etc.,  the  disposition  of  the  longitudinal  food-storing  cells  of  the 
wood  is  entirely  of  the  manner  designated  above  as  diffuse. 

In  other  orders  of  dicotyledons,  particularly  those  which  are 
with  some  degree  of  unanimity  assigned  by  systematists  to  a  high 
position,  a  strikingly  different  mode  of  parenchymatous  arrange- 
ment is  found.  This  is  notably  the  case  in  the  Compositae, 
Verbenaceae,  Oleaceae,  etc.  The  situation  in  this  type  of  distribu- 
tion of  parenchyma  can  be  clearly  indicated  by  reference  to  the 
ash.  Fig.  380  shows  the  grouping  of  the  parenchyma  at  the  end 


FIBRO VASCULAR  TISSUES:  PARENCHYMA  51 

of  the  annual  ring  and  around  the  vessels  in  this  genus.  These 
are  the  only  situations  in  which  parenchymatous  cells  are  normally 
present  in  orders  characterized  by  what  may  be  conveniently  termed 
vasicentric  parenchyma.  In  b  the  longitudinal  view  of  a  vessel  with 
its  parenchymatous  jacket  of  vasicentric  parenchyma  (that  is, 
parenchyma  confined  to  the  vicinity  of  the  vessels)  is  indicated. 
The  vasicentric  distribution  of  parenchyma  is,  other  things  being 
equal,  an  indication  of  an  advanced  systematic  position  among  the 
dicotyledons  and  is,  as  will  be  shown  in  the  sequel,  accompanied 


A  a 

FIG.  38. — Vasicentric  parenchyma  in  Fraxinus.    Explanation  in  the  text 


by  other  features  of  organization  in  the  tissues  of  the  secondary 
wood  which  are  somewhat  generally  accepted  as  indicating  a  high 
degree  of  specialization. 

In  dicotyledonous  woods,  with  either  of  the  two  characteristic 
modes  of  distribution  of  the  parenchymatous  elements  indicated 
in  the  preceding  paragraphs,  degeneracy  may  occur.  As  a  con- 
sequence the  longitudinal  elements  may  somewhat  rarely  be  ab- 
sent altogether  or  may  be  confined  to  the  end  of  the  annual  ring, 
the  latter  situation  being  much  more  commonly  found.  Fig.  39 
illustrates  the  organization  of  the  wood  in  Salix  or  Populus  (or 
equally  well  that  of  Liriodendron  or  Magnolia)  as  regards  the 


52 


THE  ANATOMY  OF  WOODY  PLANTS 


distribution  of  parenchyma tous  elements.  In  a  is  shown  the  trans- 
verse view,  and  it  is  here  clear  that  the  cells  in  question,  accentuated 
by  the  representation  of  their  walls  in  black,  are  confined  to  a  posi- 
tion at  the  end  of  the  annual  ring.  Clearly,  the  vessels  appearing 
in  the  figure  are  unaccompanied  by  any  parenchymatous  elements. 
In  b  is  represented  the  longitudinal  radial  aspects  of  the  same  wood, 
and  it  becomes  clear  that  in  this  plane,  too,  the  storage  cells  are 
confined  to  the  terminal  region  of  the  annual  ring,  for  none  can  be 
seen  in  relation  to  the  vessels.  The  situation  here  indicated  is  not 


FIG.  39. — Terminal  parenchyma  in  Popidus.     Explanation  in  the  text 

uncommon  in  the  case  of  woods  with  reduced  vasicentric  paren- 
chyma. As  in  so  many  other  instances,  the  situation  is  made  clear 
by  reference  to  more  conservative  parts,  such  as  the  root  and  the 
first  annual  ring  of  the  stem,  or  to  injured  material  showing  a 
reversion  to  the  primitive  condition.  By  such  control  of  evidence 
it  becomes  clear  that  in  the  Salicaceae,  as  well  as  in  Magnolia  and 
Liriodendron  among  the  Magnoliaceae,  the  characteristic  mode  of 
distribution  of  the  storage  cells  is  vasicentric,  for  they  occur  in  this 
manner  both  in  conservative  organs  and  parts  and  likewise  as  the 
result  of  experimental  injury.  It  cannot,  of  course,  be  too  strongly 
emphasized  in  the  case  of  comparative  anatomical  investigations 
that  a  wide  view  of  any  particular  situation  is  essential  to  an  ade- 


FIBROVASCULAR  TISSUES:   PARENCHYMA  53 

quate  comprehension  of  a  given  problem.  The  truth  of  this 
statement  will  become  more  and  more  obvious  as  a  result  of  repeated 
illustrations  in  the  sequel.  Much  more  rarely  does  the  diffuse 
condition  of  parenchymatous  disposition  in  the  dicotyledons  give 
rise  by  reduction  to  a  state  in  which  storage  elements  are  to  be 
found  only  at  the  end  of  the  annual  zones  of  growth.  This  situa- 
tion is  exemplified  by  certain  species  of  the  antarctic  beech  (Notho- 
fagus) ,  in  which,  in  contrast  to  all  the  boreal  species  of  the  Fagaceae, 
the  parenchyma  is  confined  to  the  face  of  the  summer  wood  and 
is  not  distributed  throughout  the  annual  ring,  as  is  the  rule  for 
the  family  as  a  whole.  Similar  reasoning  to  that  employed  in  the 
case  of  the  Salicaceae  and  certain  Magnoliaceae  results  in  the 
correct  conclusion  as  to  the  typical  and  primitive  mode  of  occur- 
rence of  parenchymatous  elements. 

It  is  necessary  to  emphasize  the  different  interpretations  of 
terminal  parenchyma  which  must  be  adopted  in  the  case  of  the 
conifers  and  the  dicotyledons.  In  the  coniferous  series  the  presence 
of  wood  parenchyma  on  the  face  of  the  summer  wood  is  clearly 
a  primitive  phenomenon,  both  because  of  the  comparative  and 
historical  data  and  because  of  the  equally  cogent  evidence  derived 
from  the  consideration  of  the  origin  of  parenchyma  cells  in  this 
position  in  the  conifers.  Clearly,  terminal  parenchyma  in  the 
gymnospermous  series  is  in  the  act  of  origination  in  view  of  its 
almost  imperceptible  transition  to  septate  tracheids.  In  the 
dicotyledons,  on  the  other  hand,  comparative  and  experimental 
data  alone,  hi  the  absence  at  the  present  time  of  any  adequate 
information  in  regard  to  the  historical  evolution  of  woods  of  this 
type,  lead  to  the  conclusion  that  the  occurrence  of  storage  elements 
in  the  terminal  region  of  the  annual  rings  is  rather  the  result 
of  reduction  from  a  more  elaborate  and  advanced  condition  (diffuse 
or  vasicentric)  than  one  of  primitive  simplicity. 

It  is  obvious  that  in  general  among  the  dicotyledons  we  need 
not  expect  to  have  as  clear  evidence  in  regard  to  the  problem  of  the 
derivation  of  the  longitudinal  storage  elements  of  the  wood  as  in  the 
case  of  the  gymnosperms,  and  in  particular  the  conifers,  which 
present  themselves  to  our  gaze  in  so  long  and  continuous  a  series 
in  geologic  time.  Evidence  for  the  origin  of  parenchymatous 


54 


THE  ANATOMY  OF  WOODY  PLANTS 


elements  from  the  tracheary  and  other  fibrous  constituents  of  the 
secondary  wood  in  the  dicotyledonous  angiosperms  is  sufficiently 
authentically  supplied  by  the  grouping  of  such 
elements  in  longitudinal  terminally  pointed 
groups  possessing  the  exact  configuration  of 
tracheids  or  fibers.  Fig.  40  illustrates  this 
situation  for  the  wood  of  the  alder.  To  the 
right  and  left  are  seen  fiber-tracheids  which 
constitute  the  mechanical  elements  of  the  wood 
in  this  genus.  Between  these  two  cells  lies  a 
file  of  parenchyma  tous  elements  of  the  wood. 
It  is  dear  that  the  latter,  represented  with 
heavy  black  boundaries,  form  a  series  which 
both  in  length  and  in  contour  corresponds  with 
the  adjacent  fibrous  elements.  Normally  there 
are  no  transitions  from  tracheids  to  parenchy- 
matous  cells  in  the  case  of  the  dicotyledons,  and 
even  experimental  means  do  not  in  general 
suffice  to  bring  about  the  clear  production  of 
parenchyma  as  the  result  oi  the  progressive 
septation  of  the  original  tracheary  elements  of 
the  wood.  Some  information  on  this  subject, 
however,  is  furnished  by  the  phenomena  of 
injury  in  Liquidambar  and  Prunus.  Here,  as  a 
response  to  experimental  or  accidental  injuries, 
tangential  rows  of  gum  or  mucilage  canals  are 
formed  in  the  wood.  About  these  canals  may 
sometimes  be  detected  short  elements  which 
grade  from  abbreviated  tracheids,  resulting  from 
the  septation  of  tracheids  of  normal  length,  to 
typical  parenchymatous  elements,  characterized 
by  persistent  protoplasmic  contents  and  simple 
pits  in  their  walls. 

Certain  general  characteristics  of  wood  which 
depend  as  much  as  anything  on  the  condition 
of  the  wood  Parenchyma  may  be  conveniently 
of  the  alder.  introduced  here.    Fig.  41  is  a  photograph  of  the 


FIG.  40.—  Longi- 
tudinal   view   of 


FIBROVASCULAR  TISSUES:    PARENCHYMA 


55 


polished  end  of  the  trunk  of  an  oak.  The  wood  is  clearly  separated 
into  two  regions,  a  darker  central  and  a  pale  peripheral.  The 
deeply  colored  central  region  of  the  trunk  constitutes  the  heartwood 
or  duramen.  The  uncolored  zone  which  surrounds  this  is  the 
sapwood  or  alburnum.  The  dark-hued  heartwood  is  extremely 
resistant  to  decay  and  constitutes  the  only  material  properly 
utilizable  for  exposed 
structures.  In  the 
case  of  dicotyledonous 
trees  in  general,  one 
frequently  notes  even 
with  the  naked  eye  a 
difference  in  color 
between  the  heartwood 
and  the  sapwood. 
Sometimes  this  impor- 
tant distinction  is  not 
revealed  to  the  eye, 
but  becomes  obvious 
only  under  microscopic 
investigation.  Fre- 
quently the  sap  indi- 
cates its  boundaries  in  the  felled  trunk  by  the  discoloration  brought 
about  in  its  tissues  either  by  oxidases  or  by  fungi,  or  by  both 
agencies  united.  In  conifers,  likewise,  a  distinction  between  a 
darker  central  heartwood  and  a  surrounding  pale-hued  sapwood 
can  be  often  recognized  by  the  naked  eye.  The  redwood  or 
red  cedar  presents  the  contrast  in  color  in  a  very  marked  manner. 
The  larch  and  the  spruce,  which  in  the  microscopic  organization 
of  their  woods  are  practically  identical,  can  be  readily  distinguished 
from  one  another  by  the  gross  aspect  of  their  trunks.  The  larch 
has  a  dark-brown  heartwood,  while  in  the  species  of  spruce  the 
central  region  of  the  woody  cylinder  is  in  no  way  contrasted  in 
color  with  the  peripheral  sapwood. 

It  will  be  convenient  in  connection  with  the  discussion  of  the 
parenchymatous  or  storage  elements  of  the  wood  to  elucidate 
certain  features  of  the  microscopic  organization  of  the  heartwood 


FIG.  41. — Transverse  view  of  an  oak  log  showing 
heart-  and  sapwood. 


56  THE  ANATOMY  OF  WOODY  PLANTS 

as  contrasted  with  the  sapwood.  In  this  connection  it  will  be  well 
to  begin  with  a  conifer.  Fig.  42  represents  side  by  side  the  micro- 
scopic aspects  of  heart  and  sap  tissues  in  the  case  of  the  white  pine 
(Pinus  strobus).  Beginning  with  the  sapwood,  which  appears  on 
the  right  of  the  illustration,  it  is  clear  that  both  its  rays  and  the 
cells  surrounding  a  resin  space  or  canal  are  possessed  of  nuclei  and 
likewise  contain  a  somewhat  granular  living  substance  or  proto- 
plasm. Imbedded  in  the  protoplasm  are  usually  found  oval  bodies, 


FIG.  42. — Sap-  and  heartwood  of  the  pine.    Explanation  in  the  text 

the  grains  of  starch,  which  are  lacking,  indeed,  only  in  the  cells 
immediately  surrounding  the  resin  canal  or  space.  By  examination 
of  the  bordered  pits  of  the  tracheids  it  becomes  evident  that  in  the 
wood  of  the  sap  the  membranes  of  the  pits  are  central  in  position. 
Turning  to  the  left  side  of  the  figure,  we  find  represented  the  corre- 
sponding organization  of  the  wood  in  the  case  of  the  heart.  The 
elements  distinguished  by  simple  pits  hi  the  delineation  on  the 
right — in  other  words,  the  living  cells — here  have  lost  their  living 
contents  and  are  quite  empty.  Moreover,  in  the  water-conducting 
cells,  or  tracheids,  distinguished  by  the  presence  of  bordered  pits, 
we  discover  that  the  membranes  of  the  pores  with  their  thickened 
central  region,  or  torus,  are  no  longer  median  in  position,  but  in 
general  have  become  adherent  to  one  side  or  the  other.  Further, 
the  resin  space  has  been  stopped  by  an  ingrowth  known  as  tylosis. 


FIBROVASCULAR  TISSUES:   PARENCHYMA 


57 


In  other  coniferous  woods,  particularly  those  higher  in  the  scale 
than  Pinus,  the  parenchymatous  elements,  whether  radial  or 
longitudinal  in  position,  secrete  antiseptic  substances  such  as 
essential  oils,  tannin,  etc.,  which  preserve  the  heart  structures 
from  the  decay  resulting  from  the  attacks  of  wood-destroying 
fungi.  Fig.  43  throws  light  on  the  similar  conditions  as  regards 
the  organization  of  heart  and  sap  in  the  oak,  as  an  example  of  the 
dicotyledons.  To  the  right,  radial  and  longitudinal  parenchymatous 


FIG.  43. — Heart-  and  sapwood  in  the  oak.    Explanation  in  the  text 

elements  appear  loaded  with  starch  and  provided  with  proto- 
plasm and  nucleus.  The  radial  storage  cells  are  represented  with 
light  walls,  while  the  true  wood  parenchyma  is  delineated  with 
thick,  black,  bounding  membranes.  Larger  and  smaller  vessels 
are  to  be  seen  corresponding  to  the  spring  and  summer  region  of 
the  wood.  The  rest  of  the  area  is  occupied  by  tracheids  (larger  and 
thinner-walled)  and  fiber-tracheids  (narrower  and  with  thicker 
walls).  Turning  now  to  the  left,  where  the  organization  of  the 
heartwood  is  indicated,  we  discover  the  same  absence  of  contents 
in  the  radially  and  longitudinally  directed  elements  with  simple  pits 
(in  other  words,  the  parenchymatous  elements)  as  in  the  case  of 
the  similar  structures  of  the  pine.  In  this  instance  the  larger 
vessel  presents  a  certain  analogy  to  the  resin  canal  of  the  conifer  by 
reason  of  the  fact  that  it  is  occluded  by  an  ingrowth  in  the  form  of 


58  THE  ANATOMY  OF  WOODY  PLANTS 

a  tylosis.  An  adhesion  of  the  pit  membranes  to  one  overhanging 
margin  or  the  other  of  the  pit  cannot  be  made  out.  Occasionally 
a  torus  is  present  in  the  relatively  narrow  membranes  of  the  small 
bordered  pits  of  the  water-conducting  elements  of  the  dicotyledons, 
but  it  does  not  present  the  phenomenon  of  fusion  with  the  margins 
of  the  pit  characteristic  of  heartwood  in  conifers.  The  paren- 
chymatous  constituents  of  dicotyledonous  woods  in  many  cases  give 
rise  to  highly  efficient  antiseptics  in  the  case  of  the  heartwood. 
In  many  instances,  such  as  the  oak,  the  blue  gum,  the  quebracho, 
etc.,  large  amounts  of  tannin  are  formed  which  serve  as  an  effectual 
preservative.  In  other  cases  ulmic  and  even  humic  acids  make 
their  appearance  and  exercise  greater  or  less  inhibitive  action  on 
the  organisms  which  ordinarily  bring  about  the  decay  of  woody 
structures.  In  teak  we  have  the  rare  example  of  a  structurally 
valuable  dicotyledonous  wood  which  in  the  transformation  from 
heartwood  to  sapwood  elaborates,  not  acid  substances  which  exer- 
cise a  corrosive  action  on  metals,  particularly  on  iron  and  steel, 
but  an  essential  oil.  The  enduring  heartwood  of  the  teak  (Tectona 
grandis)  is  consequently  valuable  above  all  others  for  naval  con- 
struction by  reason  of  its  compatibility  with  iron  and  steel,  since  it 
neither  corrodes  this  fundamental  structural  material  of  present 
naval  architecture  nor,  in  turn,  is  rotted  by  iron  rust. 

It  will  be  apparent  from  the  foregoing  paragraphs  that  those 
longitudinal  elements  which  subserve  the  function  of  storage  in 
the  woods  of  Mesozoic,  Tertiary,  and  actual  plants  are  of  great 
evolutionary  significance.  Their  importance  in  this  respect 
can  be  gauged  only  after  the  rays  or  radial  storage  devices  have 
been  considered  in  the  next  chapter,  and  they  will  receive  their 
final  and  fullest  appreciation  in  connection  with  the  highest  groups 
of  plants.  It  is  obvious  that  the  incentive  to  the  development  of 
longitudinal  parenchymatous  elements  was  the  appearance  of  an 
annual  winter  period  of  rest  which  in  later  geological  times,  begin- 
ning with  the  earlier  Mesozoic,  with  progressively  greater  emphasis 
marked  the  originally  unvarying  cycle  of  the  year.  The  first 
parenchymatous  elements  came  into  being,  so  far  as  our  knowledge 
at  present  goes,  in  the  earlier  Jurassic.  Their  primitive  occurrence 
was  at  the  end  of  the  annual  ring.  In  the  conifers  in  this  position 


FIBROVASCULAR  TISSUES:    PARENCHYMA  59 

they  often  clearly  reveal  their  derivation  from  tracheids  by  almost 
imperceptible  transitions  into  elements  belonging  to  this  category. 
The  terminal  situation  of  the  primitive  parenchymatous  cells  is 
probably  of  advantage  to  the  cambium  awakening  from  its  winter 
sleep  and  standing  much  in  need  of  instantly  available  food.  In 
general,  the  tangential  terminal  parenchyma  of  the  more  primitive 
conifers  has  the  same  nutritive  relation  to  the  cambial  elements  as 
the  similarly  located  tangential  pitting  of  the  tracheids  has  to  the 
water  supply  of  the  cambial  zone. 

Later  the  longitudinal  elements  devoted  to  the  function  of 
storage,  like  the  tangential  pitting  of  the  tracheids,  became  dis- 
tributed throughout  the  annual  ring.  So  long  as  there  was  no 
further  differentiation  of  the  elongated  elements  of  the  wood  there 
was  no  incentive  to  further  evolution  on  the  part  of  the  elements 
of  the  wood  parenchyma.  With  the  appearance  of  the  vessel  as 
the  final  expression  of  efficiency  in  the  transport  of  water  on  the 
part  of  the  wood  and  the  correlated  gradual  loss  of  the  aquiferous 
function  on  the  part  of  the  tracheids  (which  progressively  gave  rise 
to  fiber-tracheids  and  libriform  fibers,  respectively  more  and  more 
specialized  in  the  mechanical  direction),  a  new  tendency  found 
expression  in  the  organization  of  the  longitudinally  oriented 
storage  devices  of  the  wood.  In  the  case  of  the  higher  gymnosperms 
(Gnetales)  and  lower  dicotyledons  the  fibrous  elements  of  the  wood 
are  still  largely  capable,  by  reason  of  the  presence  of  numerous 
bordered  pits  in  their  walls,  of  the  transport  of  water.  The  higher 
dicotyledons,  however,  are  in  general  characterized  by  the  strict 
allocation  of  the  function  of  movement  of  water  to  the  vessels,  and 
the  tracheary  structures  of  lower  types  become  transformed  into 
purely  mechanical  or  partially  mechanical  and  partially  food-storing 
elements  designated  progressively  as  fiber-tracheids,  libriform 
fibers,  septate  fibers,  and  substitute  fibers.  With  the  appearance 
of  this  situation  the  parenchymatous  elements  of  the  wood 
become  more  and  more  relegated  to  the  vicinity  of  the  vessels  for 
their  necessary  supplies  of  all-important  water.  This  situation 
receives  its  final  morphological  and  evolutionary  expression  in  the 
appearance  of  strictly  localized  vasicentric  parenchyma  in  the  case 
of  high  dicotyledonous  groups,  such  as  the  Compositae,  Sapindales, 


60  THE  ANATOMY  OF  WOODY  PLANTS 

Verbenaceae,  Oleaceae,  etc.  It  must  not  be  assumed,  however, 
that  the  localization  of  the  parenchyma  about  the  vessels  is  strictly 
referable  to  the  transformation  of  the  tracheids  into  purely  mechani- 
cal elements.  It  has  been  shown  by  Miss  Holden  in  her  interesting 
investigations  on  the  Sapindales  that  it  makes  no  difference  whether 
the  fibrous  elements  of  the  wood  here  are  of  the  nature  of  tracheids 
(and  hence  are  capable  of  conducting  water)  or  are  libriform  and 
mechanical  as  regards  the  distribution  of  the  elements  of  the  wood 
parenchyma  which  in  the  group  throughout  are  vasicentric.  It  is 
thus  clear  that  the  distribution  of  the  parenchymatous  cells  in  dicoty- 
ledonous woods  is  of  morphological  importance  and  is  not  physiolo- 
gically conditioned  by  the  nature  of  the  fibrous  portions  of  the  wood, 
whether  tracheary  and  with  bordered  pits  or  libriform  and  thus 
definitely  relegated  to  purely  mechanical  functions. 

It  is  finally  important  to  note  that  not  even  the  obvious  facts 
of  distribution  can  without  proper  discrimination  be  subjected  to 
evolutionary  inference.  For  example,  in  many  dicotyledonous 
woods  terminal  parenchyma  occurs  which  might  be  regarded  as 
prima  facie  evidence  of  a  primitive  systematic  position  in  view  of 
the  situation  presented  by  the  conifers,  exhibiting  parenchyma  on 
the  face  of  the  summer  wood.  A  comparative  and  experimental 
investigation  of  this  situation  makes  it  clear  in  the  case  of  the 
dicotyledons  that  the  terminal  position  of  parenchyma  is  the  result 
of  degeneracy  either  from  the  vasicentric  or  from  diffuse  modes  of 
parenchymatous  distribution. 


CHAPTER  VI 


THE  FIBRO VASCULAR  TISSUES:    SECONDARY  WOOD— RAYS 

In  the  preceding  chapter  the  subject  of  the  origin  of  longitudinal 
storage  cells  has  been  discussed  and  to  elements  of  this  type  the 
general  appellation  of  wood  parenchyma  has  been  given.  In  the 
case  of  the  ligneous  tissues  of  vascular  plants  a  much  older  type 

of   storage   device 

exists  in  the  form  of 
radially  directed 
bands  of  masses  of 
cells  which  often,  with 
a  high  degree  of  im- 
propriety, are  desig- 
nated medullary  rays. 
This  denomination  is 
erroneous  from  the 
historical  and  evolu- 
tionary standpoint, 
since  it  is  clear  that 
in  the  first  instance 
and  in  the  earlier  and 
primitive  forms  rays 
had  no  relation  what- 
ever to  the  pith  or 
medulla  and  conse- 
quently cannot  with 
any  degree  of  pro- 
priety be  designated 
as  medullary.  Fig.  44  illustrates  the  conditions  obtaining  in  the 
case  of  the  so-called  medullary  rays  of  the  ancient  genus  Lepido- 
dendron.  The  region  of  the  pith  (p)  is  represented  largely  by  an 
empty  space,  which  is  in  turn  surrounded  by  the  primary  wood. 
This  is  distinguished  by  the  non-seriate  and  irregular  arrangement 

61 


FIG.  44. — Transverse  section  of  the  stem  of  a 
lepidodendrid,  showing  well-developed  primary  and 
secondary  wood,  the  latter  being  radially  seriate  and 
provided  with  storage  rays  (after  Scott). 


62  THE  ANATOMY  OF  WOODY  PLANTS 

of  its  cells  in  contrast  to  the  regularly  and  radially  disposed 
structures  of  the  secondary  xylem  which  lies  just  outside  the 
primary  region.  It  is  clear  from  the  figure  here  introduced  that 
the  so-called  medullary  rays  which  extend  from  the  primary  wood 
outward  cannot  be  properly  so  designated,  since  they  never  come 
in  contact  with  the  pith.  The  same  situation  occurs  in  the  stem 


FIG.  45. — Longitudinal  view  of  the  primary  wood  of  a  lepidodendrid. 
tion  in  the  text. 


Explana- 


of  many  other  extinct  fernlike  or  gymnospermous  plants  with 
secondary  growth.  It  is  worth  while  to  note,  too,  in  this  connec- 
tion, that  in  the  root  of  living  plants  in  which  the  wood  undergoes 
secondary  increase  its  radially  disposed  bands  of  storage  cells  have 
nothing  to  do  with  a  pith  or  medulla.  It  is  accordingly  evident 
from  an  examination  of  older  and  consequently  more  primitive 
forms,  and  likewise  from  the  study  of  the  organization  of  the  con- 
servative root  structure  of  living  groups,  that  the  term  medullary 


FIBROVASCULAR  TISSUES:  RAYS 


ray  is  a  misnomer.  The  most  appropriate  name  for  the  radial 
stripes  of  storage  elements  in  the  secondary  wood  is  the  merely 
descriptive  one  of  wood  ray. 

The  status  of  the  ray  having  been  preliminarily  denned  from 
the  evolutionary  standpoint  there  now  remains  the  question  of  the 
origin  of  the  hori- 
zontally directed 
bands  of  storage  tis- 
sue which  it  is  cus- 
tomary to  consider 
under  this  head.  It 
has  been  made  clear 
that  the  lepidoden- 
drids  are  the  only 
plants  which  supply 
decisive  evidence  as 
to  the  origin  of  the 
parenchyma  tous  ele- 
ments  found  uni- 
versally in  the 
primary  wood  of 
vascular  plants.  It 
will  be  well  to  recall 
the  situation  here  by 

reference  to  Fig.  45,  which  shows  a  longitudinal  view  of  the 
tracheary  and  allied  structures  of  the  first-formed  wood  of  a  speci- 
men of  the  genus  Lepidodendron  from  the  Carboniferous  of  Lanca- 
shire, England.  It  is  obvious  that,  in  addition  to  the  longer  cells 
with  reticulately  thickened  walls — the  tracheids  proper— there  are 
numerous  short  elements  with  a  similar  kind  of  sculpture.  In 
series  with  these  are  other  cells  again  which  are  quite  without  the 
usual  tracheary  thickenings  and  which  belong,  in  fact,  not  to  the 
water-conducting  system,  but  to  the  storage  category.  These  are 
wood  parenchyma.  It  is  evident  in  the  present  instance  that  the 
storage  cells  of  the  primary  wood  have  been  derived  from  what 
were  originally  tracheids  by  septation  or  division  and  subsequent 
differentiation.  It  has  been  demonstrated  in  the  previous  chapter 


FIG.  46. — Transition  from  primary  to  secondary  wood 
in  a  lepidodendrid.    Description  in  the  text. 


THE  ANATOMY  OF  WOODY  PLANTS 


that  the  parenchymatous  structures  of  the  secondary  wood  are  in 
the  first  place  derived  from  modified  tracheids. 

In  Fig.  46  is  shown  the  region  of  transition  from  primary  to 
secondary  wood  in  a  lepidodendrid,  somewhat  highly  magnified. 
The  secondary  tissue  is  most  characteristically  distinguished  by 

its  rays  running  in 
alternation  with 
bands  of  large  tra- 
cheids.  These 
radially  directed 
stripes  of  storage 
elements  at  once 
attract  attention  by 
reason  of  the  unu- 
sual organization  of 
their  component 
cells.  The  constit- 
uent units  of  the 
rays  in  this  instance 
are  reticulately 
thickened  after  the 
manner  of  tracheids 
and,  in  fact,  differ 
from  these  only  by 
their  abbreviated  length  and  the  somewhat  more  delicate  nature 
of  their  sculpture.  There  is,  indeed,  not  the  slightest  doubt  that 
in  the  case  of  the  lepidodendrids  the  rays  are  largely,  and  in  some 
instances  wholly,  composed  of  cells  belonging  to  the  category  of 
tracheids.  This  situation  is  clear  in  radial  sections  taken  length- 
wise through  the  wood.  Fig.  47  represents  such  a  section  from 
the  root  of  the  lepidodendroid  type  know  as  stigmaria.  The 
heavy  sculpture  of  the  tracheids  composing  the  mass  of  the 
wood  can  be  easily  made  out.  Running  at  right  angles  to  the 
direction  of  the  tracheary  elements  of  the  wood  are  the  cells  of  a 
ray.  These  again  show  a  considerable  degree  of  scalariform 
thickening,  only  a  few  being  completely  devoid  of  this  form  of 
sculpture.  It  is  obvious  from  the  conditions  described  in  the  case 
of  the  lepidodendrids,  a  group  of  arboreal  club  mosses  nourishing 


FIG.  47.— Radial  view  of  the  secondary  wood  of 
lepidodendrid,  showing  tracheary  origin  of  the  rays. 


FIBROVASCULAR  TISSUES:  RAYS  65 

in  the  Paleozoic,  that  rays  in  this  type  were  more  or  less  largely 
composed  of  tracheids.  It  seems  clear  for  this  reason  that  the 
radial  bands  of  storage  parenchyma  which  constitute  the  rays 
in  secondary  wood,  like  the  longitudinal  parenchyma  included 
under  the  caption  of  wood  parenchyma,  are  derived  from  the 
modification  of  tracheary  tissue.  It  thus  becomes  apparent  that 
originally  all  the  parenchymatous  constituents  of  wood,  whether 
primary  or  secondary  or  radial  or  longitudinal,  in  the  first  instance 
made  their  appearance  by  the  modification  of  tracheary  tissues. 
Tracheids,  in  fact,  constitute  the  sole  original  feature  of  organiza- 
tion in  woods,  and  the  course  of  evolution  expressing  itself  in  con- 
tinued differentiation  has  led  to  the  derivation  of  all  the  other 
features  of  ligneous  structure  from  this  primary  constituent.  In 
other  words,  wood  primitively  was  a  purely  water-conducting 
tissue,  and  the  superadded  mechanical  and  storage  functions 
subserved  by  its  organization  in  kter  geologic  times  resulted  in 
appropriate  modifications  of  the  original  tracheary  elements.  The 
derivation  of  the  rays  from  tracheary  tissues  can  be  distinguished 
clearly  only  in  the  lepidodendrids  and  their  allies.  In  others  of  the 
arboreal  cryptogams  which  were  so  characteristic  of  the  forests  of 
the  Paleozoic  age  no  evidence  of  the  origin  of  ray  cells  from  tracheids 
has  been  observed.  The  same  statement  holds  for  the  lower  and 
ancient  gymnosperms,  the  Cycadofilicales  (Pteridospermae  of  Oliver 
and  Scott)  and  the  Cordaitales  and  their  allies.  These  antique 
gymnospermous  groups,  as  well  as  the  arboreal  cryptogams  sig- 
nalized above,  had  no  storage  tissues  in  their  wood  other  than 
radial  parenchyma;  for  the  longitudinal  parenchymatous  elements 
known  as  wood  parenchyma  proper  made  their  appearance  only  in 
connection  with  the  seasonal  refrigeration  which  became  ever  more 
pronounced  during  Mesozoic  and  later  geologic  time.  The  rays 
of  the  older  plants  with  secondary  growth  were  of  two  main  types. 
In  some  instances  (e.g.,  Cycadofilicales)  they  were  composed  of 
bands  of  cells  several  elements  in  width  and  greatly  varying  in 
height.  In  the  Cordaitales  and  allied  forms  the  rays  were  ordi- 
narily uniseriate — that  is,  a  single  cell  in  width,  in  contrast  to  their 
often  multiseriate  and  considerable'  height.  In  a  general  way  it 
seems  clear  that  woods  of  the  first  type  are  perpetuated  in  the  still 
living,  although  much  reduced,  Cycadales,  while  the  Cordaitales, 


66 


THE  ANATOMY  OF  WOODY  PLANTS 


according  to  common  consent,  find  their  successors  in  the  conifers 
of  the  present  age. 

Since  it  is  highly  probable  that  the  Cordaitales  gave  origin  to 
the  Coniferales,  and  since,  as  a  consequence,  they  present  us  with 


FIG.  48.— View  of  cordaitean  wood  in  three  dimensions.    Explanation  in  the  text. 

the  starting-point  of  the  radial  parenchyma  of  the  latter,  it  will  be 
well  to  consider  the  organization  of  this  group  in  respect  to  the 
structure  of  the  rays.  Fig.  48  shows  the  structure  of  a  cordaitean 
wood.  The  tracheids  in  this  genus  are  usually  extremely  long; 


FIBROVASCULAR  TISSUES:  RAYS  67 

consequently  only  a  portion  of  their  length  is  represented  in  the 
figure.  The  pitting  is  somewhat  characteristic  and  is  ordinarily 
marked  by  the  large  number  of  pores  and  their  consequent  crowded 
and  alternating  arrangement.  The  rays  cross  the  direction  of  the 
longitudinal  elements  and  are  characterized  by  their  thin  walls, 
which  are,  however,  clearly  pitted  laterally  in  relation  to  the  radial 
walls  of  the  tracheids.  It  is  to  be  noted  in  passing  that  there  are 
no  tangential  pits  on  the  walls  of  the  longitudinal  elements  of  the 
wood — a  feature,  as  has  been  indicated  in  a  previous  chapter,  very 
generally  characteristic  of  the  woods  of  the  gymnosperms  of  the 
Paleozoic  regardless  of  their  affinities.  In  the  transverse  section 
of  the  wood  the  rays  stand  out  distinctly  as  uniseriate  files  of  cells, 
having  their  axes  radially  elongated.  The  rays  are  in  lateral 
communication  with  the  tracheids  by  half-bordered  pits.  Other- 
wise their  thin  walls  are  not  characterized  by  the  presence  of  pores. 
The  tracheary  elements  of  the  wood  are  tangentially  in  com- 
munication by  numerous  radial  pits,  but  their  tangential  walls  are 
quite  free  from  pores,  so  that  in  these  ancient  gymnosperms  water 
could  make  progress  in  the  trunk  only  in  a  tangential  direction. 
The  tangential  view  of  the  wood  shows  the  radial  pits  of  the  tra- 
cheids as  well  as  the  lateral  ones  of  the  cells  of  the  medullary  rays  in 
profile  view. 

After  the  discussion  of  the  organization  of  rays  in  the  Cordaitales 
we  are  in  a  favorable  position  to  understand  the  condition  in  the 
conifers.  In  this  connection  it  will  be  well  to  start  with  the  most 
complicated  condition  in  living  representatives  of  the  group,  since 
the  coniferous  gymnosperms,  as  will  be  shown  clearly  in  a  subse- 
quent chapter,  constitute  a  reduction  series  with  the  more  complex 
forms  at  the  bottom  and  those  with  simpler  organization  at  the 
top.  Fig.  49  represents  radial  and  tangential  views  of  the  wood  of 
the  white  pine  (Pinus  strobus).  Taking  first  the  radial  view  shown 
in  a,  a  number  of  important  contrasts  in  organization  to  cordaitean 
woods  are  to  be  seen.  First  of  all  as  regards  the  tracheids,  or  rather 
such  part  of  them  as  is  included  in  the  field  of  view,  it  is  clear  that 
they  are  distinguished  from  similar  structures  in  the  Cordaitales 
by  the  smaller  number  of  pits  and  the  considerably  larger  size  of 
these.  Further,  the  pits  in  face  view,  instead  of  presenting  the 


68 


THE  ANATOMY  OF  WOODY  PLANTS 


merely  double  contour  of  the  more  ancient  group,  are  marked  by 
triple  concentric  outlines.  The  outer  circle  corresponds  to  the 
boundary  of  the  pit  membrane,  while  the  inner  one  outlines  the 
mouth  or  aperture  of  the  pit.  The  intermediate  circular  outline 
delimits  the  thickened  central  region  of  the  torus,  a  structure, 
which  so  far  as  is  known,  was  not  present  in  Paleozoic  gymnosperms. 
Its  presence  in  the  conifers  is  in  all  probability  correlated  in  some 


A  D 

FIG.  49.— Radial  and  tangential  sections  of  the  wood  of  the  white  pine  (Pinus 
Strobtts).    Explanation  in  the  text. 

way  with  the  large  size  of  the  bordered  pits;  it  has,  in  fact,  as 
pointed  out  in  an  earlier  chapter,  been  interpreted  as  a  safety 
device  useful  in  preventing  the  rupture  of  the  broad  pit  membranes 
under  extreme  pressure.  Not  only  are  the  pits  of  large  size  and 
more  complex  organization  in  the  pine  than  in  cordaitean  forms, 
but  the  walls  of  the  tracheary  elements  in  their  vicinity,  partic- 
ularly toward  the  ends  of  the  tracheids,  are  distinguished  by  trans- 
verse bands  of  cellulosic  or  pectocellulosic  material,  and  these  are 
conveniently  designated  "bars  of  Sanio."  They  should  not  be 
confused  with  the  trabecttlae,  radially  directed  lignified  bars  running 
transversely  through  the  lumina  of  the  cells  of  many  gymnosperms 
living  and  extinct.  (They  are  even  found  in  some  cases  among  the 


FIBROVASCULAR  TISSUES:  RAYS  69 

angiosperms.)  There  is  good  reason  to  believe  that  the  "bars  of 
Sanio"  are  an  original  and  characteristic  feature  of  the  organization 
of  the  wood  of  the  Coniferales  and  allied  groups.  The  tracheids 
shown  in  the  figure  differ  from  those  of  the  cordaitean  forms  by 
their  periodic  variation  in  size,  those  laid  down  in  the  beginning  of 
the  annual  increment  being  of  larger  caliber  than  those  coming 
into  existence  toward  its  close.  The  late  tracheids  are  distinguished, 
as  indicated  in  earlier  pages,  by  their  tangential  pits  seen  in  profile 
in  the  radial  section.  Turning  our  attention  now  to  the  ray 
itself,  we  see  at  once  from  the  figure  that  a  considerably  greater 
degree  of  complication  is  present  than  that  exemplified  by  the 
similar  structure  in  the  case  of  the  Cordaitales.  Manifestly  the 
radial  elements  are  of  two  kinds.  First  there  are  the  cells  which 
constitute  the  central  region  of  the  ray  and  which  in  life  are  char- 
acterized by  protoplasmic  and  other  contents.  This  situation  is 
indicated  by  the  copious  simple  pits  which  ornament  the  vertical 
and  horizontal  walls  of  the  cells.  The  strong  pitting  is  also  a  clear 
feature  of  difference  from  the  Cordaitales  where  the  walls  of  the 
ray  cells  are  in  general  thin  and  unpitted.  Laterally  the  central 
elements  of  the  rays  are  related  to  the  tracheids  by  means  of  very 
large,  somewhat  angular  pits.  The  second  type  of  element  char- 
acteristic of  the  ray  in  Pinus  is  likewise  distinguished  by  the  nature 
of  its  pitting.  All  the  pits  seen  on  the  walls,  whether  horizontal, 
vertical,  or  lateral  (related  to  the  tracheids),  belong  to  the  bordered 
type.  The  cells  of  the  ray  possessing  this  peculiar  organization 
are  typically  marginal  in  position  and,  naturally,  are  quite  without 
protoplasmic  contents,  except  in  the  early  stages  of  development. 
Such  tracheary  elements  in  coniferous  rays  are  commonly  desig- 
nated marginal  tracheids  or  simply  marginal  cells.  They  obviously 
permit  the  easy  movement  of  water  in  the  radial  direction  in  those 
woods  characterized  by  their  presence. 

In  the  tangential  view  presented  in  b,  Fig.  49,  it  is  clear  that 
there  are  two  types  of  rays — namely,  narrow  uniseriate  ones  and 
broader  ones  tapering  at  either  end  to  the  uniseriate  condition. 
The  former  are  known  as  linear  rays  and  the  latter  as  fusiform  rays. 
It  is  evident  from  the  figure  that  the  ray  of  greater  width  is  char- 
acterized by  the  presence  of  a  partly  occluded  cavity,  a  resin  canal. 


7o 


THE  ANATOMY  OF  WOODY  PLANTS 


The  stopping  up  of  the  resin  canal  is  explained  by  the  fact  that  the 
section  is  taken  from  the  heartwood,  which,  as  has  been  pointed 

out  in  a  former  chapter, 
is  distinguished  by  the 
phenomenon  of  tylosis 
or  occlusion  of  the  secre- 
tory spaces  by  means  of 
ingrowths  of  the  resi- 
niferous  parenchyma. 
In  the  case  of  the  nar- 
row or  linear  ray  it  is 
possible  to  distinguish 
in  the  tangential  view, 
by  means  of  their  char- 
acteristic pitting,  both 
the  parenchymatous 
central  cells  and  the  tra- 
cheary  marginal  ele- 
ments. The  central 
elements  with  their 
simple  pits  are  in  rela- 
tion to  air  spaces  in  the 
angles.  The  presence  of 
both  linear  and  fusiform 
rays  is  a  constant  fea- 
ture of  organization  in 
the  pine  and  its  nearer 
allies.  The  linear  rays 
are  doubtless  an  older 
feature  than  the  larger 
ones  which  contain  the 
horizontal  resin  canals, 
since  they  resemble  most 

FIG.  50. — Tangential  view  of  the  wood  of  the  J 

Pseudotsuga.    Explanation  in  the  text.  nearly  the  radial  paren- 

chymatous  structures  of 

the  Cordaitales.     The  fusiform  rays  serve  to  bring  about  a  connec- 
tion between  the  vertical  resin  canals  in  different  annual  rings,  not 


FIBROVASCULAR  TISSUES:  RAYS 


only  with  one  another,  but  often  with  the  similar  but  larger  secre- 
tory cavities  which  are  present  in  the  bark. 

Fig.  50  illustrates  the  relation  between  the  vertical  and  hori- 
zontal resin  canals  as  seen  in  a  vertical  section  of  the  wood  of  the 
Douglas  fir  (Pseudotsuga).  To  the  left  of  the  center  lies  a  fusiform 
ray  with  its  included  resin  space,  which  opens  broadly  on  the  right 
into  a  vertical  secretory  _____^ _ 
canal.  The  plane  of  sec- 
tion happens  to  lie  near 
the  end  of  the  annual  ring 
so  that  the  cellular  struc- 
tures lying  in  view  are 
almost  entirely  parenchy- 
matous.  The  lining  of 
the  resiniferous  spaces  is 
evidently  largely  com- 
posed of  elements  with 
thick  walls  and  bordered 
pits,  a  condition  very  com- 
monly present  in  the 
representatives  of  the 
Pineae  other  than  Pinus 
itself. 

After  the  discussion  of 

the  pine  and  its  nearer  allies  we  may  proceed  with  advantage  to 
the  description  of  the  ray  structures  in  coniferous  woods  of  simpler 
organization.  Fig.  51  illustrates  the  structure  of  the  ray  in  the 
wood  of  the  balsam  fir  (Abies  balsamea).  It  is  clear  that  in  this 
case  the  cells  of  the  rays  are  all  of  one  kind  and  that  the  marginal 
tracheids  are  conspicuous  by  their  absence.  The  parenchymatous 
elements  which  compose  the  rays  in  the  genus  under  considera- 
tion are  in  relation  to  one  another  on  both  vertical  and  hori- 
zontal walls  by  numerous  simple  pits.  The  lateral  walls  show 
bordered  pores  such  as  ordinarily,  except  in  certain  species  of 
Pinus,  characterize  the  relation  between  rays  and  tracheids. 
The  organization  of  the  linear  rays  in  conifers  other  than  the 
Abietineae  is  in  general  of  a  simple  nature;  and  in  the  higher 


FIG.  51.— Ray  of  the  balsam  fir.     Explana- 
tion in  the  text. 


THE  ANATOMY  OF  WOODY  PLANTS 


subtribes,  such  as  the  Cupressineae,  Taxodineae,  and  Taxineae, 
even  the  intercommunicating  simple  pits  of  the  horizontal  and 
vertical  walls  of  the  ray  cells  are  clearly  and  often  conspicuously 
absent.  In  such  cases,  as  is  to  be  expected,  the  wall  of  the  ray 
elements  is  in  general  thinner  and  often  curved. 

An  interesting  condition  of  organization  of  the  radial  paren- 
chyma is  presented  by 
Chamaecy parts  nootkaten- 
sis.  Here,  as  is  shown  in 
Fig.  52,  the  rays  are  fre- 
quently marked  by  the 
presence  of  tracheary  ele- 
ments on  one  or  both 
margins.  The  illustration 
represents  the  radial 
aspect  of  the  transition 
from  summer  to  spring 
wood,  and  the  features  of 
structure  are  in  general 
such  as  one  would  expect 
to  find  under  the  circum- 
stances, except  as  regards 
ray  organization.  The 
normal  presence  of  margi- 
nal tracheids  in  the  species  figured  gains  a  special  evolutionary  sig- 
nificance from  the  fact  that  similar  conditions  are  found  in  other 
representatives  of  the  Taxodineae,  Cupressineae,  and  the  genus 
Abies  among  the  Abietineae  as  a  consequence  of  injury.  For 
reasons  which  will  be  fully  discussed  in  a  subsequent  chapter  it 
seems  quite  clear  that  the  structures  which  are  found  to  appear  as 
a  result  of  injury  in  vascular  plants  with  secondary  growth  are  often 
of  the  nature  of  reversions  to  an  ancestral  condition.  Fig.  53  illus- 
trates in  radial  view  the  wood  formed  after  injury  in  a  root  of  the 
Big  Tree  (Sequoia  gigantea) .  Parts  of  two  annual  rings  are  included, 
and  the  ray  clearly  shows  features  of  structure  which  are  abnormal 
for  the  Taxodineae.  To  the  left  below  and  in  the  spring  wood  can 
be  seen  several  cells  included  in  the  substance  of  the  ray  which  are 


FIG.  52. — Ray  of  the  Nootka  cypress, 
scription  in  the  text. 


De- 


FIBROVASCULAR  TISSUES:  RAYS 


73 


clearly  tracheary  in  their  character,  since  they  contrast  with  the 
adjoining  elements  both  in  the  absence  of  protoplasmic  contents 
and  in  the  occurrence  of  bordered  pits  in  all  their  walls.  It  is 
evident  that  in  the  case  under  discussion  short  tracheids  may  make 
their  appearance  among  the  elements  of  the  radial  parenchyma  as 
a  sequel  to  injury.  The  interesting  investigations  of  Miss  Holden 


FIG.  53. — Ray  from  the  injured  root  of  Sequoia  glgantea.  Explanation  in  the 
text  (after  Holden). 

on  the  Cupressineae  and  Taxodineae  as  a  whole  make  it  clear  that 
abnormalities  of  this  kind  in  these  subtribes  of  coniferous  gym- 
nosperms  are  a  common  feature  in. the  wood  formed  after  injury. 

It  follows  from  the  statements  and  illustrations  in  connection 
with  the  last  paragraph  that  normally  in  Chamaecyparis  noot- 
katensis  and  traumatically  in  practically  all  representatives  of  the 
Cupressineae  and  Taxodineae,  ray-tracheids  are  found  such  as 
are  a  feature  of  the  normal  structure  of  the  wood  in  the  lower 
members  of  the  Abietineae.  The  most  natural  interpretation  of 
this  phenomenon  is  in  connection  with  the  biological  doctrine  of 


74 


THE  ANATOMY  OF  WOODY  PLANTS 


reversion.  This  general  doctrine  will  receive  particular  considera- 
tion at  a  later  stage  and  consequently  need  not  be  elucidated  here. 
If  the  radial  tracheids  occurring  under  the  conditions  described  in 
the  foregoing  paragraph  are  interpreted  as  a  reversion  to  an  ances- 
tral condition,  it  follows  that  the  simple  type  of  ray  found  in  Abies 
and  in  the  Cupressineae  or  Taxodineae  is  by  no  means  primitive, 

but  is  the  result  of 
simplification  from 
the  more  complex 
state  of  organiza- 
tion of  the  ray, 
characteristic  of 
the  lower  living 
Abietineae.  Obvi- 
ously an  experi- 
mental as  well  as  a 
purely  anatomical 
investigation  of 
ray  structures  is 
necessary  for  their 
complete  morpho- 
logical and  evolu- 
tionary under- 
standing. 

Not  only  does 

one  find  in  the  case  of  certain  coniferous  woods  of  simpler 
organization  evidence  of  derivation  from  ancestral  types  present- 
ing the  complication  of  marginal  ray-tracheids,  but  likewise  in  the 
genus  Cedrus  among  the  Abieteae,  which  has  normally  only 
linear  or  uniseriate  rays,  fusiform  radial  structures  containing 
horizontal  resin  canals  are  found.  This  situation  is  made  clear 
by  Figs.  54  and  55,  which  present  tangential  views  of  the  normal 
and  injured  wood  of  Cedrus  Libani  (the  cedar  of  Lebanon).  In  the 
traumatic  or  injured  wood  of  cedar,  which,  so  far  as  the  geological 
record  supplies  us  with  definite  evidence,  is  the  oldest  representative 
of  the  Abieteae  or  firlike  conifers,  we  have  clearly  indicated  a 
condition  ensuing  from  injury  which  definitely  unites  the  cedar 


FIG.  54. — Tangential  section  of  the  normal  wood  of 
Cedrus  Libani. 


FIBRO VASCULAR  TISSUES:   RAYS 


75 


with  the  genus  Pinus,  much  older  geologically  and  more  complicated 
in  the  normal  organization  of  the  wood.  It  will  be  clear  from  the 
statement  in  this  connection  that  fusiform  rays  have  also  an  impor- 
tance from  the  experimental  standpoint,  quite  comparable  with 
marginal  tracheids,  but  less  copiously  expressed  in  traumatic 
phenomena. 

The  rays  of  the 
Cycadales  and 
their  allies  present 
no  features  of  spe- 
cial evolutionary 
interest,  so  far  at 
any  rate  as  is 
known  at  the  pres- 
ent time;  they  may 
therefore  be  dis- 
missed with  the 
simple  statement 
that  they  are  typi- 
cally multiseriate 
bands  and  not  the 
linear  structures 
constituting  the 
primitive  condition 
of  organization  of  the  radial  parenchyma  for  the  Cordaitales, 
Coniferales,  and  Ginkgoales.  The  ray  structures  in  the  highest 
gymnosperms,  the  Gnetales,  are  best  discussed  in  connection  with 
the  similar  features  of  dicotyledonous  woods,  which  they  resemble 
in  so  many  respects.  This  procedure  is  the  more  desirable  because 
the  living  Gnetales  are  represented  by  a  very  small  number  of 
genera  of  widely  separated  geographical  ranges. 

At  this  point  the  dicotyledonous  angiosperms  may  appropriately 
be  considered  in  regard  to  the  organization  of  their  rays.  Fig.  56 
reproduces  a  transverse  section  of  the  wood  of  the  oak.  The  struc- 
ture in  this  case  is  highly  complicated  and  corresponds  to  a  marked 
development  of  the  principle  of  division  of  labor.  The  movement 
of  water,  the  functions  of  strength  and  of  storage,  are  all  distinctly 


FIG.  55.— Tangential  section  of  the  wood  of  Cedrus 
Libani  formed  after  injury. 


76 


THE  ANATOMY  OF  WOODY  PLANTS 


allocated  to  particular  and  highly  differentiated  categories  of  cells. 
In  the  present  connection  we  are  concerned  only  with  the  struc- 
tures included  under  the  heading  of  radial  parenchyma.  Clearly 
the  rays  in  the  oak  are  of  two  types,  even  as  seen  in  transverse 
section.  A  small  number  are  very  broad  and  constitute  a  large 
bulk  of  storage  tissue.  In  contrast  to  these  in  respect  both  to  size 
and  to  number  are  linear  rays,  abundantly  present  in  the  figure. 

It  is  best  in  this 
instance  to  focus 
our  attention  on 
the  composition  of 
the  uniseriate  or 
linear  rays  and 
their  relation  to 
the  various  ele- 
ments represented 
in  the  complex 
organization  of  the 
wood  in  the  same 
genus,  Quercus. 

Fig.  57  repre- 
sents the  uniseriate 
rays  in  radial  and 
tangential  aspect. 
In  a  is  seen  the 
tangential  view. 
The  ray  is  obviously  composed  of  cells  which  are  all  alike  and 
related  to  one  another  and  to  the  air  spaces  by  simple  pits.  Half- 
bordered  pits  connect  ray  cells  with  tracheids.  In  b  and  c  are 
shown  radial  aspects  of  the  ray  in  relation  to  the  various  structural 
elements  of  the  wood.  On  the  left  and  right  in  b  files  of  vertical 
parenchyma  cross  its  course  and  are  related  to  the  radial  elements 
by  groups  of  clustered  pits.  To  the  left  of  the  middle  of  b  is  shown 
a  vessel  (this  type  of  element  will  be  considered  in  the  following 
chapter)  which  communicates  with  the  radial  storage  elements  by 
large,  generally  oval,  pits.  The  rest  of  the  width  of  b  is  occupied 
by  tracheids,  which  in  turn  communicate  with  the  cells  of  the 


FIG.  56. — Transverse  section  of  the  wood  of  the 
red  oak. 


FIBRO VASCULAR  TISSUES:  RAYS 


77 


rays  by  small  bordered  pits.  In  c  is  seen  a  region  where  the  linear 
ray  passes  through  a  purely  fibrous  portion  of  the  wood;  and 
here  there  are  no  pits  at  all,  since  the  mechanical  elements  which, 
as  has  been  indicated  in  a  former  chapter,  have  been  differen- 
tiated from  the  tracheids  no  longer  supply  water  to  the  other 
structures  of  the  wood.  It  will  be  clear  from  the  foregoing 
account  'that  the  uniseriate  or  linear  rays  of  the  oak  are  of  uni- 
form and  simple  organization  as  regards  their  constituent  ele- 
ments, but  that  these  are  characterized  by  a  variety  of  pitting 
corresponding  to  the  high  degree  of  differentiation  of  the  wood 


FIG.  57. — Longitudinal  and  transverse  views  of  linear  rays  in  the  oak.    Explana- 
tion in  the  text. 


through  which  they  pass.  The  large  rays  of  the  oak  can  better 
be  considered  at  a  later  stage  after  a  type  showing  a  more  general- 
ized condition  of  radial  organization  has  been  examined. 

It  will  be  convenient  and  profitable  to  consider  in  the  present 
connection  the  genus  Casuarina,  which  occurs  in  the  East  Indian 
and  Australasian  regions,  since  here  we  find  in  various  species  all 
the  main  types  of  organization  of  the  wood  rays  exemplified  by  the 
dicotyledons.  First  is  presented  the  tangential,  longitudinal  view 
of  the  wood  in  Casuarina  Fraseri.  Here  the  structural  conditions 
are  manifestly  very  similar  to  those  found  in  the  oaks  of  northern 
latitudes,  for  there  are  two  distinctly  contrasted  categories  of  rays — 
namely,  the  numerous  linear  or  uniseriate  and  the  sparse  broad 
rays.  In  comparison  with  the  wood  of  C.  Fraseri,  presenting  a 
strong  resemblance  to  that  of  a  white  or  black  oak  (Fig.  58),  is 
that  of  C.  torulosa,  shown  in  Fig.  59.  Here  the  linear  rays  are  as 


THE  ANATOMY  OF  WOODY  PLANTS 


in  C.  Fraseri,  but  the  large  band  of  radial  parenchyma  is  obviously 
not  homogeneously  parenchymatous,  but  is  separated  into  pointed 
groups  by  the  presence  of  interspersed  fibers.  In  other  words, 
instead  of  a  continuous  mass  of  storage  tissue  there  is  present 
an  aggregation  of  rays  of  a  certain  size,  separated  from  one  another 
by  strands  of  fibers.  This  condition  of  the  large  ray  may  be 
conveniently  designated  as  aggregate.  In  Fig.  60  is  represented 

the  tangential 
aspect  of  the  wood 
in  C.  equisetifolia. 
In  this  type  we  no 
longer  see  a  sharp 
contrast  between 
large  multiseriate 
or  aggregate  rays 
and  small  entirely 
uniseriate  ones,  but, 
as  it  were,  a  more 
democratic  organi- 
zation of  the  radial 
parenchyma,  in 
which  no  extremely 
large  radial  paren- 
chymatous masses . 
are  found,  as  all 
grade  almost  imper- 
ceptibly into  one  another  in  size.  The  last-described  condition 
of  the  rays,  for  reasons  to  be  indicated  later,  is  here  designated 
as  diffuse.  We  have  thus  in  the  single  genus  Casuarina  three 
distinct  types  of  radial  parenchyma:  first,  the  northern  oak  type 
in  which  huge  rays  stand  in  the  sharpest  contrast  to  narrow  uni- 
seriate bands;  secondly,  a  condition  in  which  the  contrast  still 
obtains  as  regards  the  dimensions  of  the  rays,  with  the  dis- 
tinction that  the  large  masses  are  not  homogeneous  but  pene- 
trated by  bands  of  fibers;  and,  finally,  there  is  to  be  noted  a  state 
or  organization  in  which  all  rays  are  of  moderate  width  and  are 
scattered  somewhat  evenly  throughout  the  tangential  or  transverse 


FIG.    58. — Tangential   section   of    the   wood   of 
Casuarina  Fraseri.    Explanation  in  the  text. 


FIBROVASCULAR  TISSUES:  RAYS 


79 


section  of  the  wood.  In  the  first  condition  the  large  rays  are 
known  as  compound;  in  the  second  they  are  designated  as  aggre- 
gate, and  in  the  third  state,  where  there  is  generally  no  marked 
superiority  in  size  and  the  rays  intergrade  almost  imperceptibly, 
they  are  known  as  diffuse. 

Of  the  three  conditions  of  organization  of  radial  parenchyma 
in  the  dicotyledons  described  above,  the  compound  is  extremely 
rare  in  trees  but  is 
commonly  found  in 
climbing  and  her- 
baceous types.  The 
diffuse  condition  of 
the  radial  paren- 
chyma is  very  com- 
mon in  forest  trees, 
but  is  much  less 
characteristic  of 
plants  of  herba- 
ceous texture.  The  \  Hu !);'!!, 
genus  Casuarina 
has  purposely  been 
chosen  for  the 
exemplification  of 
the  problems  con- 
nected with  the 
evolution  of  radial 

parenchyma  in  the  dicotyledons,  not  because  it  is  necessarily  a 
primitive  form,  but  because  it  shows  the  situation  synoptically 
and,  moreover,  furnishes  very  clear  evidence  as  to  the  relation 
of  the  various  types  to  one  another  from  the  evolutionary 
standpoint. 

An  exposition  of  the  interesting  and  important  situation  of  the 
ray  structures  in  the  dicotyledons  can  best  be  approached  by  a 
diagrammatic  comparison  with  the  conditions  presented  by  the 
conifers.  Fig.  61  reproduces  the  essential  features  of  organization 
of  a  coniferous  stem  with  whorled  leaves — for  example, *a  Juniperus 
or  a  Cupressus.  The  leaves,  three  in  number,  are  indicated  in 


FIG.  59. — Tangential  section  of   the  wood  of  Casu- 
arina torulosa.    Explanationin  the  text. 


8o 


THE  ANATOMY  OF  WOODY  PLANTS 


black  on  the  periphery  of  the  stem.  Centrally  placed  is  the 
pith,  surrounded  by  the  woody  cylinder,  which  in  turn  is  circled 
by  the  phloem  and  the  cortex.  The  woody  tissues  are  encroached 
^^  upon  by  three  deep 

bays  extending 
from  the  medulla 
and  subtended  by 
the  three  leaves. 
These  extensions 
from  the  pith  mark 
the  presence  of  the 
leaf  gaps,  interrup- 
tions in  the  con- 
tinuity  of  the 
woody  cylinder  re- 
lated to  the  passing 
.out  of  the  foliar 
traces.  Later  the 
intervals  in  the 
wood  are  covered 
by  the  activity  of 
the  cambium,  so 


FIG.  60. — Tangential  section  of  the  wood  of  Casua- 
rina  equisetifolia.     Explanation  in  the  text. 


that  the  cylinder  becomes  continuous 
in  the  second  or  third  year  of  growth. 
Clearly  there  is  no  structural  feature 
of  importance  in  the  woody  cylinder 
related  to  the  leaf  trace  other  than  the 
foliar  gap.  This  is  the  general  situa- 
tion in  the  case  of  coniferous  stems  as 
well  as  in  that  of  their  Paleozoic  ances- 
tors, the  Cordaitales. 

Having  diagrammatically  compassed 
the  organization  of  the  stem  in  the  coni-        FlG'  6i.-Diagrammat' 

.  .  transverse  section  of  a  coniferous 

fers,  we  are  in  a  position  to  consider  the    twig.    Explanation  in  the  text, 
case  of  such  a  dicotyledon  as  Casuarina. 

In  Fig.  62  a"re  reproduced  the  essential  features  of  topography  of 
a  small  branch  in  this  genus.    Leaves,  as  in  the  case  of  the  conifer- 


FIB RO VASCULAR  TISSUES:  RAYS 


8 1 


ous  diagram  given  previously,  are  represented  in  black  on  the 
periphery  of  the  stem.  The  central  pith  is  likewise  encircled  in 
turn  by  wood,  phloem,  and  cortex.  Here,  too,  there  are  excur- 
sions of  the  pith  at  six  points,  extending  into  the  second  annual 
increment  of  the  wood;  these  subtend  radially  six  corresponding 
leaves.  In  the  diagram  under  discussion  there  are,  however,  two 
marked  features  of  contrast  to  those  presented  in  the  foregoing 
scheme  of  a  coniferous  axis.  First  of  all,  the  wood  is  character- 
ized by  the  presence  of  vessels,  and 
secondly  by  a  broad  radial  stripe 
which  extends  from  each  leaf  gap 
outward.  This  radial  stripe  con- 
trasts with  the  rest  of  the  woody 
cylinder  by  the  absence  of  vessels 
and  the  clustering  of  rays.  The 
radial  parenchymatous  stripes  which 
lie  in  the  region  of  the  six  radial 
bands  corresponding  to  six  leaves  are 
not  only  more  prominent  than  the 
linear  and  somewhat  sparse  rays  in 
the  rest  of  the  wood  but  are  of 

greater  width.  The  aggregations  of  rays  related  to  the  leaves 
shown  in  the  diagram  are,  in  fact,  clustered  rays  of  the  type  exem- 
plified by  C.  torulosa  (Fig.  59)  and  are,  as  a  consequence,  aggregate 
rays.  Since  they  are  in  this  instance  clearly  related  to  leaves, 
they  may  at  the  same  time  be  appropriately  designated  foliar 
aggregate  rays. 

With  the  exposition  of  the  essential  features  of  organization 
present  in  coniferous  and  dicotyledonous  stems,  respectively,  we 
are  in  a  position  to  proceed  further  with  the  highly  important 
discussion  of  the  evolution  of  the  radial  parenchymatous  structures 
of  dicotyledonous  woods.  Simplicity  will  be  served  and  ambiguity 
avoided  if  in  further  elaboration  we  hold  to  the  conception  of  the 
rays  as  they  present  themselves  in  a  small  twig  of  a  few  years' 
growth.  Fig.  63  illustrates  synoptically  the  main  types  of  rays  in 
the  dicotyledons  as  seen  in  small  branches.  The  x>nly  essential 
departure  from  the  conditions  in  nature  is  the  delineation  of  the 


FIG.  62. — Diagrammatic  trans- 
verse section  of  the  stem  in  Casua- 
rina.  Explanation  in  the  text. 


82 


THE  ANATOMY  OF  WOODY  PLANTS 


three  important  categories  of  rays  as  occurring  side  by  side  in  the 
same  stem.  In  the  center  of  the  circle  representing  diagram- 
matically  a  dicotyledonous  stem  is  figured  a  leaf  gap  and 


B' 


FIG.  63. — Synoptical  diagram  representing  the  transverse  and  longitudinal 
topography  of  the  rays  related  to  the  leaves  in  species  of  Casuarina.  Explanation 
in  the  text. 

peripherally  its  subtending  leaf.  Between  the  two  lies  a  corre- 
sponding ray  of  the  aggregate  type.  It  is  to  be  noted  here  that 
there  is  a  modification  of  the  rate  of  growth  of  the  annual  rings 


FIBRO VASCULAR  TISSUES:   RAYS  83 

in  the  region  of  the  aggregate  ray  which  results  in  their  being 
locally  depressed. 

To  the  right  is  to  be  seen  another  leaf  with  its  corresponding 
gap  and  ray.  In  this  case  the  ray  structure  where  it  is  still  near 
the  leaf  trace  (solid  black)  is  the  same  as  in  that  just  described, 
namely,  aggregate.  Farther  out,  however,  the  components  of  the 
aggregate  ray,  instead  of  maintaining  their  original  relations  to  one 
another,  begin  to  diverge  in  the  tangential  plane.  At  the  same  time 
vessels  which  are  conspicuous  by  their  absence  while  the  ray  is  in 
the  aggregate  condition  begin  to  appear  in  the  widening  strands  of 
wood  which  separate  the  diverging  rays.  This  process  continues 
and  becomes  more  and  more  marked  in  successive  outer  annual 
rings.  The  final  result  is  that  what  was  once  a  congery  or  aggre- 
gation of  .rays  separated  from  one  another  by  purely  fibrous  strands 
becomes  a  more  and  more  diffuse  cluster  of  rays  separated  by  ever- 
widening  vascularized  intervals  of  wood.  In  this  condition  the 
original  aggregation  of  rays  not  only  becomes  diffuse  in  a  fanlike 
fashion  in  the  outer  region  of  the  woody  cylinder,  but  the  individual 
rays  subdivide,  thus  accentuating  the  condition  of  diffusion.  The 
phenomenon  of  the  subdivision  of  the  rays  for  the  sake  of  simplicity 
is  omitted  in  the  diagrammatic  representation.  It  is  clear  that  the 
appearance  of  the  conditions  depicted  in  the  ray  to  the  right  of  the 
diagram  (at  b)  in  the  case  6f  all  the  foliar  rays  of  a  stem  would 
result  in  a  diffusion  of  rays  of  a  medium  breadth  throughout  the 
older  wood — in  other  words,  to  the  condition  shown  in  Fig.  60  for 
the  adult  wood  of  Casuarina  equisetifolia  or  an  allied  species. 

Turning  now  to  the  left  of  the  diagram  (at  c\  we  observe  a 
foliar  or  leaf  ray  of  still  another  type.  Here,  as  in  the  diffuse  con- 
dition of  the  foliar  ray  represented  in  b,  the  original  state  is  that  of 
aggregation  with  the  exclusion  of  vessels,  a  situation  which  is  per- 
manent in  the  type  diagrammed  at  a.  In  the  later  annual  rings  in 
this  type  the  aggregation  becomes  a  homogeneous  mass  of  paren- 
chyma by  the  disappearance  of  the  fibrous  strands  which  separate 
the  components  of  the  aggregation  or  congery  from  one  another. 
Where  the  clusters  become  fused  into  large  homogeneous  bands  of 
storage  tissue  by  the  parenchymatous  transformation  of  the  original 
separating  fibers  of  the  aggregation,  the  result  is  the  compound  ray 


84  THE  ANATOMY  OF  WOODY  PLANTS 

represented  at  c  in  our  diagram.  This  condition  is  present  in  the 
adult  structure  of  the  wood  of  Casuarina  Fraseri,  as  is  shown  in 
Fig.  58.  The  primitive  aggregate  condition  persists,  on  the  other 
hand,  in  C.  tondosa,  as  is  shown  in  Fig.  59. 


FIG.  64. — Diagrammatic  representation  of  a  twig  of  Picea  canadensis.  Explana- 
tion in  the  text. 

In  the  longitudinal  aspect  of  the  diagram  presented  to  the  reader 
are  shown  the  vertical  views  of  the  three  types  of  rays  illustrated 
horizontally  in  a,  b,  c.  Fig.  630'  shows  the  tangential  topog- 
raphy of  an  aggregate  ray.  The  clustering  of  the  masses  of  radial 


FIBROVASCULAR  TISSUES:    RAYS 


parenchyma  and  the  exclusion  of  vessels  can  be  readily  seen.  At 
b'  is  indicated  the  tangential  projection  of  the  diffuse  condition 
of  rays.  Here  the  original  cluster  has  become  scattered  and 
vessels  are  now  present  among  the  rays.  In  c'  is  shown  the  longi- 
tudinal tangential  aspect  of  the  compound  ray — -that  is,  the  con- 
dition in  which  the  original  aggregation  has  become  fused  into  a 
large  solid  mass  of  radial  parenchyma. 

The  conditions 
in  a  diagram  of  a 
coniferous  stem  in 
three  dimensions 
may  now  be  dis- 
cussed. Fig.  64 
illustrates  the 
topography  of  a 
two-year-old  twig 
of  Picea  canaden- 
sis.  In  the  trans- 
verse aspect  the 
pith  surrounded 
by  xylem,  phloem, 
and  cortex  can  be 
distinguished. 
Projecting  from 
the  surface  are  the 
leaves  or  their  bases.  The  outline  of  the  pith  is  indented  by  a  num- 
ber of  bays,  which  are  the  deeper  the  nearer  they  are  in  the  vertical 
plane  to  the  departing  trace  of  a  leaf.  In  relation  to  one  of  these  on 
the  side  of  the  pith  nearer  the  observer  is  an  actual  trace  running 
horizontally  in  the  wood.  The  transverse  aspect  of  the  wood 
shows  the  presence  of  numerous  narrow  rays.  The  face  of  the 
stem  facing  the  observer  is  cut  away  to  show  the  topographical 
relations  of  the  leaf  traces  in  the  wood.  It  is  clear  that,  contrary 
to  the  conditions  observed  in  the  stem  of  Casuarina  diagrammati- 
cally  represented  in  Fig.  62,  there  are  no  modifications  in  the 
grouping  of  the  rays  or  in  other  features  with  reference  to  the 
foliar  traces  (appearing  as  oval  dots). 


FIG.  65. — Transverse  section  of  a  twig  of  Casuarina  Fraseri 


86 


THE  ANATOMY  OF  WOODY  PLANTS 


It  will  be  advantageous  as  a  sequel  to  the  representation  of  the 
prominent  types  of  rays  in  the  dicotyledons  in  diagram  to  view 
them  in  actual  photographs  in  the  case  of  the  genus  Casuarina.  We 
may  profitably  begin  here,  as  in  the  case  of  the  diagrams,  with 
transverse  sections.  Fig.  65  reproduces  the  cross-section  of  a 
small  twig  of  Casuarina  Fraseri.  The  leaves  are  in  the  main  still 
present  in  depressions  on  the  surface  of  the  stem  and  clearly  sub- 
tend the  foliar 
rays.  Rather  nar- 
row leaf  gaps  pene- 
trate  the  first 
annual  ring  and 
are  twice  as  numer- 
ous as  the  append- 
ages at  a  given 
node  (that  is,  there 
are  twelve  gaps, 
although  only  six 
appendages). 
This  duplication 
of  the  gaps  is  due 
to  the  fact  that  the 
whorls  of  leaves 
alternate  at  differ- 
ent nodes,  and  the 
gaps  correspond- 
ing to  these  are  persistent,  with  the  natural  result  of  gaps  twice 
as  numerous  as  the  appendages.  Fig.  66  shows  part  of  the 
foregoing  much  more  highly  magnified.  At  the  top  is  a  persistent 
leaf  and  in  the  middle  line  below  lies  the  pith,  which  is  sending  off 
an  extension,  the  leaf  gap;  and  this,  on  reaching  the  second  annual 
ring,  undergoes  considerable  enlargement.  Between  the  wide 
termination  of  the  leaf  gap  in  the  beginning  of  the  second  annual 
ring  and  in  line  with  and  subtending  the  leaf  on  the  outside  of  the 
stem  lies  the  foliar  ray.  The  magnification  is  not  sufficient  in  the 
figure  to  show  the  organization  of  the  leaf  ray;  hence  a  more 
enlarged  representation  is  introduced  in  Fig.  67.  Here  it  is  clear 


FIG.  66. — Part  of   transverse  section  of  a 
Casuarina  Fraseri  more  highly  magnified. 


FIBRO VASCULAR  TISSUES:   RAYS 


that  the  structure  of  the  ray  in  the  twig  in  C.  Fraseri  is  different 
from  the  adult  condition,  shown  in  Figs.  58  and  63,  for  in  the 
younger  axis  the  ray  is  obviously  penetrated  by  fibers,  and  these 
are  absent  in  the  adult.  The  truth  of  this  statement  will  become 
still  more  apparent  by  reference  to  Fig.  68,  which  reproduces  the 
tangential  aspect  of  the  wood  in  a  somewhat  older  branch  of  the 
same  species.  The  prominent  mass  of  radial  storage  'tissue  in 
the  center  is  the 
foliar  ray.  It  is  dis- 
tinctly fibrous  and 
is  consequently  still 
in  the  condition  of 
aggregation.  As 
the  stem  thickens 
the  fibers  are  grad- 
ually transformed 
into  parenchyma- 
tous  elements  more 
and  more  like  the 
cells  of  the  ray. 
Thus  it  is  that  the 
compound  ray  of 
C.  Fraseri  comes 
into  being.  It  is 
evident  that  its 
early  condition  is 
one  of  aggregation, 

and  that  this  is  followed  by  a  gradual  transformation  into  the 
compound  state  by  the  fusion  of  the  originally  separate  members 
of  the  aggregation. 

The  photographic  representations  lead  likewise  to  conclusions 
in  harmony  with  the  diagrammatic  figures  in  the  case  of  the  diffuse 
condition  of  the  foliar  ray.  It  will  not  be  necessary  to  introduce 
a  total  general  and  a  partial  more  detailed  view  of  the  twig  in  this 
instance,  since  the  topographical  relations  are  practically  the  same 
as  thos'e  shown  in  the  case  of  C.  Fraseri.  Fig.  69  illustrates  the 
situation  in  the  diffusion  of  the  foliar  ray  as  exemplified  by 


FIG.  67. — Portion  of  Fig.  66  still  more  highly  magni- 
fied to  show  organization  of  the  foliar  ray.  Explanation 
in  the  text. 


88 


THE  ANATOMY  OF  WOODY  PLANTS 


C.  stricta.  It  is  evident  from  the  photograph  that  a  mass  of  ray 
tissues  on  the  left  (representing  the  end  nearer  the  pith),  charac- 
terized by  moderate  breadth  and  the  absence  of  vessels,  passes 
toward  the  right  (outer  side  topographically)  into  a  continually 
widening  fanlike  cluster  of  rays  among  which  vessels  become  more 
and  more  prominent.  Fig.  70  shows  a  tangential,  longitudinal 
section  of  the  wood  in  a  small  branch  of  C.  equisetifolia.  Here  is 

present  an  aggrega- 
tion of  rays,  foliar 
in  its  character, 
which  as  yet  has 
scarcely  begun  to 
diverge  into  the 
diffuse  condition 
and  consequently 
includes  no  vessels 
in  its  substance. 
C.  stricta  and  C. 
equisetifolia  are 
both  species  with 
the  diffuse  type  of 
ray  in  the  adult 
wood.  C.  stricta 
has  been  chosen  to 
illustrate  the  trans- 
verse aspect  of  the 
diverging  rays  in 

the  diffuse  type  only  because  the  larger  size  of  the  radial  bands 
in  this  species  make  the  topographical  relations  more  obvious. 
The  difference  in  gross  appearance  between  stems  with  aggre- 
gate or  compound  rays  on  the  one  hand  and  diffuse  rays  on 
the  other  hand,  is  very  striking.  This  is  well  illustrated  by 
Fig.  700.  On  the  left  is  seen  a  polished  segment  of  the  trunk 
of  C.  Fraseri  (compound  and  aggregate  rays).  On  the  right 
appears  a  polished  trunk  of  C.  stricta  (diffuse  rays).  Here 
the  rays  are  at  first  distinct  and  later  die  out.  The  conspicu- 
ous stage  is  aggregate,  while  that  representing  the  disappearance 


FIG.  68. — Tangential  section  of  a  large  ray  in  the 
younger  stem  of  Casuarina  Fraseri,  showing  aggregate 
condition. 


FIBRO VASCULAR  TISSUES:   RAYS  89 

of  noteworthy  bands  of   radial    parenchyma  marks   the   diffuse 
condition. 

It  has  been  demonstrated  in  the  case  of  Casuarina  that  there 
are  three  main  types  of  radial  parenchyma  in  the  secondary  wood 
— namely,  the  aggregate,  the  diffuse,  and  the  compound.  Of 
these  three  types  the  aggregate  is  manifestly  the  oldest,  and  the 
other  two  have  originated  from  it  by  divergence  in  the  diffuse 


FIG.  69. — Transverse  section  of  aggregate  rays  diverging  into  the  diffuse  condi- 
tion in  Casuarina  stricta. 


condition  and  by  fusion  in  that  state  designated  compound. 
What  is  diagrammatically  clear  in  Casuarina  for  the  radial  paren- 
chymatous  structures  is  much  less  obvious  in  most  other  dicotyle- 
dons, and  in  no  case  are  the  relations  so  well  shown  as  in  the  genus 
named.  Among  arboreal  forms  the  oak  is  of  interest  in  exhibiting 
both  the  aggregate  and  the  compound  condition  of  the  rays.  In 
this  genus  the  species  occurring  in  warm  regions  are  ordinarily 
characterized  by  the  possession  of  clustered  or  aggregate  large 
rays  in  contrast  to  the  uniseriate  or  linear  rays  present  in  the  mass 
of  the  wood.  In  species  of  northern  climates  the  rays  are  solid  or 
compound  in  their  nature,  but  even  here  the  condition  of  aggregation 


9o 


THE  ANATOMY  OF  WOODY  PLANTS 


is  found  in  the  young  plant  as  a  passing  phase  and  as  frequently 
persisting  for  many  years  in  that  most  conservative  of  plant  organs, 

the  root.  In  the  Betu- 
laceae,  by  contrast,  the 
diffuse  type  of  wood  ray 
prevails,  and  it  is  likewise 
clearly  derived  from  a 
primitive  state  of  aggre- 
gation. In  the  birch,  for 
example,  diffuse  rays 
more  usually  distinguish 
the  stem,  while  the  aggre- 
gate type  is  often  found 
present  in  the  case  of  the 
root.  The  general  ana- 
tomical conditions  as  well 
FIG.  70.— Tangential  section  of  young  twig  as  experimental  data  in 

of  Casuarina  equisetifolia.  showing  an  aggregate         ,1-1^  j    T>   *. 

foliar  ray  in  Center  flanked  on  either  side  by        the    Fagaceae   and    BetU- 

uniseriate  rays.  laceae  justify  the  conclu- 


FIG.  700. — Photographs  of  transverse  views  of  polished  stems  of  Casuarina  stricta 
(right)  and  Casuarina  Fraseri,  showing  the  general  topography  of  diffuse  and  compound 
rays.  The  diffusing  rays  of  C.  stricta  appear  to  die  out,  while  the  rays  of  the  other 
species  become  not  only  accentuated  but  also  clearly  more  numerous. 


FIBROVASCULAR  TISSUES:  RAYS  91 

sion  that  wherever  the  compound  or  diffuse  types  of  rays  are 
present  in  members  of  these  groups  they  have  been  derived  from 
an  earlier  condition  of  aggregation.  The  type  of  ray  structure 
is  not  strictly  related  to  any  particular  organization  of  the  flower, 
since  amentaceous,  archichlamydeous,  and  metachlamydeous  fami- 
lies present  to  an  almost  equal  extent  the  various  categories  of 
rays.  In  a  general  way  it  may,  however,  be  stated  that  the  diffuse 
condition  of  the  rays  is  characteristic  of  arboreal  forms,  while  the 
compound  condition  occurs  mainly  in  vines  and  herbs. 


CHAPTER  VII 
THE   FIBROVASCULAR   TISSUES:     SECONDARY  WOOD— VESSELS 

The  vessel  is  an  element  of  structure  which  in  the  higher  forms 
and  in  the  secondary  wood  is  of  extremely  great  evolutionary 
importance.  Before  we  take  up  this  type  of  ligneous  element  in 
the  seed  plants  it  will  be  well  to  consider  its  occurrence  in  lower 
forms  and  in  the  primary  wood.  It  was  pointed  out  by  De  Bary 
many  years  ago  that  many  of  the  scalariform  tracheids  of  the 
bundles  in  the  bracken  fern  are  of  the  nature  of  vessels,  using  that 
term  to  apply  to  elements  which  are  not  merely  pitted  but  actually 
perforated  at  the  ends,  thus  permitting  a  much  more  ready  passage 
of  water.  The  statements  of  the  distinguished  anatomist  of 
Strassburg  have  been  confirmed  and  denied  by  more  recent  inves- 
tigators, but  there  seems  to  be  no  doubt  whatever  on  the  basis 
of  our  improved  technique  that  he  was  in  every  respect  correct. 
Fig.  'jia  illustrates  the  organization  of  a  vessel  in  Pteris  aquilina 
isolated  from  the  surrounding  tracheids  by  maceration.  It  is 
clear  that  the  scalariform  bars  are  separated  from  one  another  by 
wider  intervals  at  the  ends  of  the  element  and  that  the  bars  them- 
selves are  considerably  more  slender  in  this  region  and  are  not 
distinguished  by  as  pronounced  overhanging  margins  as  is  the  case 
in  the  similar  structures  on  the  lateral  walls.  In  b  is  represented 
a  profile  view  of  the  terminal  inclined  wall  of  the  tracheid,  indicat- 
ing plainly  the  absence  of  the  membrane  which  characterized  the 
lateral  scalariform  pits.  Further,  the  margin  or  border  in  the 
case  of  the  terminal  open  pits  is  slightly  developed  in  comparison 
with  that  of  the  lateral  pores  with  membranes  intact.  Conditions 
simila/  to  those  found  in  the  bracken  appear  in  many  other  repre- 
sentatives of  the  Filicales  and  are  by  no  means  confined  to  the 
Polypodiaceae,  which  at  the  present  time  not  only  are  the  largest, 
but  also  are  considered  in  many  respects  the  most  specialized  family 
of  the  group.  Elements  resembling  vessels  in  the  presence  of 
terminal  perforations  have  likewise  been  described  for  the  genus 

92 


FIBRO VASCULAR  TISSUES:  VESSELS 


93 


Selaginella  among  the  lycopodineous  forms.  It  is  thus  clear  that, 
so  far  as  the  primary  structures  of  the  wood  are  concerned,  vessels 
are  present  even  in  the  case  of 
representatives  of  the  lower  vas- 
cular plants. 

Although  in  certain  ferns 
and  lycopods  structures  occur 
which,  physiologically  at  any 
rate,  represent  vessels,  these 
cannot  be  regarded  as  quite  on 
the  same-  morphological  and 
evolutionary  footing  as  the  ves- 
sels of  the  highest  gymnosperms 
and  the  angiosperms.  The  step 
from  a  tracheid  to  a  vessel,  where 
all  the  tracheids  are  scalariform, 
as  is  the  case  with  the  tracheids 
of  ferns  and  lycopods,  is  a  much 
shorter  one  than  when  the 
fibrous  elements  are  pitted  and 
not  scalariform.  We  find  as  a 
consequence  that,  although  ves- 
sels or  elements  which  have  been 
regarded  as  such  occur  low  in 
the  scale  of  the  Vasculares  in 
the  primary  structures,  they 
make  their  appearance  in  the 
secondary  wood  only  in  the 
higher  representatives  of  the 
seed  plants. 

The  secondary  wood  of  the 
extinct  Paleozoic  arboreal  cryp- 
togams, although  of  ten  perfectly          FIG.  71.— Face  and  profile  views  of 
preserved,  in  no  authentic     the  %*f  ?f  ^essf  found  in  the  primary 

wood  of  the  bracken. 

instance  has  yet   revealed   ele- 
ments which  may  with  any  degree  of  propriety  be  designated  as 
vessels.     The   same   situation  obtains   in   the  case  of  the  lower 


94 


THE  ANATOMY  OF  WOODY  PLANTS 


gymnosperms  and  even  in  the  higher  representatives  of  the  group 
as  far  up  as  the  conifers.  It  is  in  the  Gnetales,  the  most  advanced 
forms  among  the  naked-seeded  Spermo- 
phyta,  that  vessels  first  make  their  appear- 
ance in  the  cylinder  of  the  secondary 
xylem.  Fig.  720  represents  a  smaller  ele- 
ment of  this  type  from  the  wood  of  the 
genus  Ephedra.  The  walls  terminating 
the  structure  under  discussion  are  dis- 
tinctly at  angles  to  the  lateral  ones  and 
are  remarkable  for  the  extremely  large 
pits  which  cover  their  surfaces.  The  pits 
of  the  terminal  walls  are  not  only  very 
much  larger  than  the  lateral  pits,  but, 
with  two  or  three  exceptions,  they  have 
lost  their  membranes.  In  the  pores  where 
the  membranes  still  persist  a  well-marked 
torus  reveals  its  presence  in  face  view. 
The  pits  of  the  lateral  walls  of  the  tra- 
cheids  in  the  case  of  Ephedra  are  not  very 
much  narrower  than  those  of  the  terminal 
surfaces,  but  invariably  are  closed  by 
membranes  thickened  centrally  by  a  well- 
marked  torus.  Fig.  726  shows  a  some- 
what larger  vascular  element  from  the 
wood  of  Ephedra,  which  has  its  enlarged 
terminal  pit  entirely  perforate  as  a  result 
of  the  complete  disappearance  of  the  mem- 
branes. In  Fig.  73  is  shown  a  vessel  in 
which,  as  an  exceptional  condition  for  the 
genus  under  discussion,  there  is  a  tendency 
to  fusion  on  the  part  of  the  enlarged  termi- 
nal pit.  Although  in  the  Gnetales  the 
phenomena  of  fusion  of  the  end  pits  of 
the  vessels  is  rare,  it  becomes  the  rule  in 
the  angiosperms,  as  will  be  shown  at  a  later  stage.  At  a  appears 
a  vessel  which  is  transitional  from  an  element  of  this  type  to  a 


FIG.  72. — Smaller  and 
larger  vessels  of  Ephedra. 


FIBROVASCULAR  TISSUES:  VESSELS 


95 


tracheid.  It  is  distinguished  by  the  fact  that  its  end  walls  do 
not  make  a  definite  angle  with  the  lateral  ones  but  taper  grad- 
ually. It  is  further  peculiar  in  the  circumstance  that  its  terminal 
pits  are  imperforate  with  one  exception  and  have  their  mem- 
branes marked  by  the  presence  of  a  very  distinct  torus.  In  c  is 
figured  a  tangential  view  of  a  vessel  in  Ephedra,  showing  the 
enlarged  terminal  pores  in  profile  and  making  it  clear  that  in  this 
case,  as  in  Pteris  aquilina,  the  vessels  become  patent  by  the  dis- 
appearance of  the  membranes  of  the  pits.  The  organization  of  the 


FIG.  73. — Vessels  of  Ephedra  (after  Thompson).     Explanation  in  the  text 

tracheae  or  vessels  in  the  two  remaining  genera  of  the  Gnetales — 
namely,  Welwitchia  and  Gnelum — is  not  essentially  different  from 
that  described  for  Ephedra,  and  consequently  need  not  be  considered 
here.  The  Gnetales  as  a  whole  are  distinguished,  not  only  by  the 
presence  of  vessels  in  their  wood,  but  by  the  possession  of  rays  of 
the  angiospermous  type,  as  has  been  indicated  in  the  preceding 
chapter. 

The  angiosperms  are  characterized  throughout  by  the  presence 
of  vessels,  generally  of  a  high  type,  but  in  some  cases  showing  clear 
indications  of  derivation  from  tracheids.  It  is  only  in  certain 
xerophytic  genera  among  the  Magnoliaceae  and  in  certain  Cactaceae 
and  Crassulaceae  that  these  t  characteristically  angiospermous 


96 


THE  ANATOMY  OF  WOODY  PLANTS 


elements  are  absent  in  the  dicotyledons.  No  case  is  at  present 
known  of  their  default  in  monocotyledons.  There  are  two  main 
modifications  of  vascular  elements  or  tracheae  in  the  angiosperms — 
namely,  vessels  with  scalariform  perforations  and  those  with 
porous  perforations.  The  term  perforation  is 
applied  to  the  actual  apertures  which  make 
their  appearance  in  the  terminal  or  other  walls 
of  typical  vessels.  The  first-named  condition 
of  perforation  is  characteristic  of  lower  groups 
and  lower  genera  among  the  dicotyledons,  while 
the  second  is  usually  found  in  the  case  of  the 
higher  orders  (the  Compositae,  for  example)  of 
the  dicotyledons  and  seems  to  be  universal  for 
the  monocotyledons. 

In  Fig.  74  is  represented  a  vessel  of  the 
lower  type — that  is,  one  in  which  the  perfora- 
tions in  the  terminal  wall  are  scalariform.  This 
is  an  element  from  the  root  of  the  alder.  The 
lateral  confines  of  the  vessel  are  covered  with 
small  bordered  pits  which  in  this  particular  case 
are  arranged  in  a  somewhat  regularly  banded 
fashion  as  a  result  of  the  contact  of  the  con- 
ducting element  under  discussion  with  the  cells 
of  a  ray  of  the  wood.  If  the  relation  had  been 
with  tracheids  or  other  vessels,  the  arrangement 
of  the  pores  would  not  be  so  regular.  Pits  are 
clearly  seen  in  profile  on  the  sides  of  the  vessels, 
indicating  the  fact  that  water-conducting  ele- 
ments belonging  to  this  category  do  not  neces- 
sarily have  their  lateral  pitting  confined  to 
the  radial  walls.  The  ends  of  the  vessel  are 
strongly  inclined  and  are  manifestly  at  definite 
angles  to  the  lateral  walls.  The  illustration 
presents  the  vascular  element  from  the  radial  view;  hence  it  is 
obvious  in  this  case  that  the  perforated  end  walls  are  radial.  This 
is  a  situation  seldom  departed  from  in  the  wroods  of  dicotyledonous 
trees.  In  other  words,  a  radial  section  of  a  dicotyledonous  wood 


FIG.  74.— Vessel 
from  Alnus,  showing 
the  scalariform  type 
of  perforation. 


FIBROVASCULAR  TISSUES:  VESSELS 


97 


ordinarily  reveals  the  face  view  of  the  characteristically  perforated 
terminal  walls  of  the  vessel.  We  find  that  it  is  distinguished  by 
the  presence  of  open  slits  which  are  in  general  without  borders  and 
by  smaller  or  larger  clearly  bordered  pits.  The  latter  structures 
retain  their  membranes  and,  where  they  become  extremely  elon- 
gated, are  clearly  the  result  of  the  fusion  of  two  or  more 
horizontally  approximated 
bordered  pits.  Fig.  7  5 ,  which 
represents  part  of  the  termi- 
nal wall  somewhat  diagram- 
matically  and  on  a  larger 
scale,  shows  the  mode  of 
fusion  of  pits  and  also  shows 
that  the  final  result  of  this 
process,  when  a  number  of 
horizontally  seriate  pits  are 
concerned,  is  the  formation 
of  an  elongated  slit  which 
reveals  its  primitive  nature 
only  by  the  retention  of  bor- 
ders at  the  ends.  The  slit  as 
a  whole  not  only  has  lost  its 
borders,  but  likewise  the  mem- 
brane of  the  row  of  fused  pits 

has  disappeared.  It  is  thus  evident  that  the  slits  which  occur 
between  the  horizontal  bars  of  the  lattice  work  in  the  terminal 
walls  of  a  vessel  of  this  type  are  the  result  of  the  fusion  of  rows  of 
pits,  accompanied  by  a  simultaneous  loss  of  pit  membranes. 

Before  we  leave  the  subject  of  the  mode  of  origin  of  the  per- 
forations which  characterize  the  terminal  walls  of  vessels  of  the 
lower  type  in  the  angiosperms  it  will  be  well  to  discuss  the  situa- 
tion in  another  group  which  shares  with  the  true  Amentiferae  in 
the  minds  of  students  of  evolution  the  claim  to  be  primitive  repre- 
sentatives of  the  dicotyledons.  In  Fig.  760  is  shown  a  vessel  from 
the  root  of  Liriodendron  Tulipifera.  In  this  instance  there  is  no 
sharp  differentiation  between  terminal  and  lateral  walls,  although 
in  general  in  the  genus  such  a  distinction  is  clearly  present.  In 


FIG.  75. — Diagram  showing  the  origin 
of  scalariform  perforations  by  the  fusion  of 
pits  in  the  end  wall  of  the  vessel. 


98 


THE  ANATOMY  OF  WOODY  PLANTS 


passing  from  the  upper  part  of  the  figure  downward  there  is  a 
transition  from  bordered  pits  arranged  in  horizontal  rows  to  per- 
forations of  somewhat  larger  dimensions  which  are  obviously 
derived  from  bordered  pits  by  the  disappearance  of  borders  and 
membrane.  The  truth  of  this  statement  can  be  inferred  from 
the  fact  that  some  of  the  perforations  still  retain  more  or  less 
of  the  original  bordered  condition,  particularly  along  their  lateral 
margins.  Fig.  766  shows  another  vessel  from  the  root  of  the  same 


FIG.  76.— Vessels  from  root  of  Liriodendron  Tidipifera  (a  and  b) ,  c,  vessel-like 
elements  formed  after  injury  in  the  non-vascular  magnoliaceous  genus  Drimys. 

genus  in  which  the  situation  is  somewhat  different.  Here  the  pits 
are  scalariform,  as  in  the  tracheary  elements  of  the  Pteridophyta, 
but  with  an  important  distinction  which  is  not  always  kept  in 
mind  in  evolutionary  speculations.  In  the  ferns  and  allied  forms 
scalariform  elements  are  a  primitive  feature  of  organization  of  the 
wood,  while  in  the  secondary  wood  of  the  angiosperms  the  scalari- 
form sculpture  of  the  lateral  walls  is  an  exclusive  feature  of  the 
vessels  and  is  the  result  of  the  lateral  fusion  of  horizontal  rows  of 
pits  and  consequently  cannot  in  any  way  be  regarded  as  a  primitive 
condition  of  structure  as  in  the  lower  forms.  Proceeding  from  top 
to  bottom,  as  in  a,  we  find  in  this  case  the  same  loss  of  borders  and 
membranes  leading  to  the  appearance  of  perforations  of  the 
scalariform  type.  The  structure  in  question,  in  fact,  exactly 


FIBROVASCULAR  TISSUES:  VESSELS  99 

simulates  the  vessel  of  Pteris  aquilina  figured  in  connection  with 
an  earlier  paragraph.  Although  the  result  is  the  same,  the  manner 
of  reaching  it  is  different  in  the  two  cases,  and  a  distinction  should 
of  course  be  made  in  drawing  any  evolutionary  conclusions.  It 
will  be  clear  to  the  reader  from  the  evidence  presented  in  the 
present  and  in  a  former  paragraph  that  the  vascular  structures  of 
the  lower  dicotyledons  do  not  originate  as  in  the  Gnetales  simply 
by  enlargements  of  pits  and  the  disappearance  of  the  pit  membranes 
in  the  terminal  regions  of  the  vessels.  On  the  contrary,  they 
typically  take  their  origin  as  a  consequence  of  the  lateral  fusion 
of  horizontal  rows  of  pits  with  a  correlated  disappearance  of  the 
membranes.  The  vessel  in  the  lower  angiosperms  is,  however,  as 
clearly  a  derivative  of  the  tracheid  as  it  is  in  the  case  of  the  highest 
of  the  gymnosperms.  Further,  in  the  angiosperms  the  vessel  or 
trachea,  as  a  result  of  its  much  more  complex  mode  of  evolution,  is 
more  distinct  from  the  fibrous  or  tracheary  element  than  it  is  in 
any  of  the  lower  groups. 

The  type  of  vessel  characteristic  of  the  higher  dicotyledons  and 
the  mass  of  monocotyledons  may  now  profitably  occupy  our 
attention.  Fig.  77  illustrates  three  vessels  belonging  to  the 
category  of  elements  with  porous  end  walls  (that  is,  vessels  with 
so-called  porous  perforations).  In  a  is  shown  such  an  element 
from  the  oak  in  approximately  radial  aspect.  The  terminations 
of  the  vessel  taper  and  are  distinguished  by  the  large  aperture  or 
pore.  The  lateral  surfaces  are  covered  with  pits,  which  are  of  two 
kinds.  The  smaller  ones,  provided  with  a  distinct  border,  put  the 
vessel  in  relation  with  other  similar  structures  and  with  tracheids. 
The  simple  and  slightly  irregular  pits  represented  in  about  the 
middle  horizontal  region  indicate  the  presence  of  a  small  ray  in 
contact  with  the  vascular  element.  In  b  is  represented  a  vessel 
from  the  wood  of  the  poplar  in  radial  view.  The  same  slanting 
ends  and  large  terminal  pores  as  are  present  in  the  oak  similarly 
characterize  the  vessel  of  Populus.  The  uniform  crowded  lateral 
pitting  indicates  that  the  face  of  the  vessel  presented  to  the  observer 
was  in  contact  with  another  vessel.  In  c  is  presented  the  somewhat 
tangential  view  of  a  vessel  in  the  maple.  The  ends  of  the  vascular 
element  do  not  differ  essentially  from  those  of  the  oak  and  poplar. 


100 


THE  ANATOMY  OF  WOODY  PLANTS 


In  this  case  the  wall  to  the  left  has  been  in  contact  with  a  ray,  as 
evidenced  by  the  grouping  of  the  pits.  The  region  of  the  lateral 
wall  to  the  right  has  been  in  contact  with  another  vessel  and  is 


FIG.  77. — Various  types  of  vessels  in  the  dicotyledons.    Explanation  in  the  text 

conspicuous,  not  only  by  reason  of  the  characteristic  arrangement 
of  the  pits,  but  also  by  the  presence  of  a  somewhat  spiral  internal 
sculpture,  the  so-called  tertiary  thickening,  which  always  marks 
(when  present)  the  contact  or  "party  wall"  between  vessel  and 
vessel.  Those  surfaces  of  the  vascular  element  which  are  in  contact 


FIBROVASCULAR  TISSUES:  VESSELS 


with  the  libriform  fibers  are  devoid  of  both  pits  and  spiral  internal 
sculpture. 

It  will  be  profitable  to  consider  next  the  type  of  vessel  charac- 
teristic of  the  older  wood  of  the  stem  of  Lirioden- 
dron.  This  category  of  vessel  is  covered  with  pits 
in  horizontal  rows,  indicating  contact  with  another 
vascular  element.  Below  and  above  the  sharply 
sloped  end  walls  are  seen;  these  are  neither 
porous,  as  in  the  case  of  the  vessels  of  the  last 
figure,  nor  marked  by  the  simultaneous  presence 
of  narrow  scalariform  perforations  and  bordered 
pits,  as  in  the  rootwood  of  Magnolia  and  in  the 
general  secondary  wood  of  many  of  the  Betu- 
laceae,  etc.  The  terminal  apertures  of  the  vessel, 
in  fact,  are  crossed  by  a  few  remote  bars.  The 
type  of  vascular  element  characteristically  present 
in  the  old  wood  of  Liriodendron  serves,  indeed,  as 
an  intermediate  stage  between  the  vessel  with  true 
scalariform  perforations  and  that  with  porous 
terminal  apertures. 

The  truth  of  this  assertion  becomes  manifest 
from  a  consideration  of  Fig.  79,  which  represents 
vascular  structures  from  the  wood  of  the  high  blue- 
berry, Vaccinium  corymbosum.  In  a,  b,  and  c  are 
shown  successive  transitions  from  end  walls  with 
many  pits  and  few  narrow  scalariform  perforations 
to  fewer  pits  and  more  scalariform  apertures,  and 
to  a  third  condition  where  the  pits  have  dis- 
appeared and  only  somewhat  wide  scalariform 
openings  remain.  In  d  and  e  appear  further 
stages  in  which  the  bars  separating  the  terminal 
large  slits  become  fewer  and  more  degenerate  until 
finally  a  simple  porous  condition  is  reached.  It  is 
thus  clear  that  the  vessel  with  porous  perforations 
is  a  further  elaboration  of  that  with  scalariform 
perforations,  just  as  this  in  turn  has  taken  its  origin  from  a  vascular 
element  with  pitted  perforations  of  the  nature  found  in  the  Gnetales. 


FIG.  78.— Ves- 
sel from  the  stem 
of  Liriodendron. 


THE  ANATOMY  OF  WOODY  PLANTS 


The  hypothesis  that  the  scalariform  vessel  is  the  forerunner  of  the 
porous  one  is  also  confirmed  by  a  consideration  of  the  first-formed 
region  of  the  woody  cylinder.  In  the  oak,  for  example,  as  well  as 
in  many  other  instances,  although  vessels  with  porous  terminal 
apertures  are  characteristic  of  the 
mature  wood,  elements  of  vascular 
nature  with  scalariform  perforations 
are  commonly  present  in  the  first 
annual  ring,  particularly  in  the  vicin- 
ity of  the  protoxylem.  It  is  thus  evi- 
dent that  the  vascular  structures  in 
dicotyledonous  woods  supply  a  valu- 
able argument  for  the  general  validity 
of  the  hypothesis  of  organic  evolution. 
The  lateral  walls  of  the  vessels 
may  next  receive  further  considera- 
tion. It  has  been  indicated  in  a 
former  paragraph  that  the  side  walls 
of  a  vessel  have  their  sculptural 
features  largely  determined  by  the 
nature  of  the  adjacent  cells  of  the 
wood.  Where  vessels  are  in  contact 
with  rays,  the  communicating  pits 
are  characterized  by  a  grouping  and 
seriation  corresponding  to  the  outlines 
and  direction,  whether  transverse  or 
longitudinal,  of  the  ray  cells.  Where 
the  relation  is  with  other  vessels  or 
with  true  tracheids,  the  pits  are 
numerous  and  likewise  arranged  in  a 
somewhat  definite  manner.  If  merely 
mechanical  elements  abut  on  the 

vascular  walls,  pitting  is  quite  absent.  Further,  if  so-called 
tertiary  spirals  are  present  on  the  inner  side  of  the  walls  of  the 
vascular  elements,  these  are  confined  to  those  vertical  regions  in 
contact  with  other  vessels  or  with  tracheids. 


FIG.  79. — Vessel  of  V actinium 
corymbosum,  illustrating  the 
origin  of  the  porous  type  of 
perforation  from  the  scalariform. 
Explanation  in  the  text. 


FIBROVASCULAR  TISSUES:    VESSELS 


103 


Where  vessel  comes  in  contact  with  vessel,  we  find  frequently  a 
mode  of  pitting  characteristic  of  a  genus  or  even  a  family.  Details 
in  this  connection  are  beyond  the  range  of  a  work  so  elementary 
as  the  present  one,  but  a  few  prominent  features  may  be  indicated. 
As  has  been  shown  in  the  figure  of  the  vessel  of  Liriodendron  above 
(Fig.  76) ,  lateral  pits  may  be  in  horizontal  rows.  In  other  instances, 
as  in  the  oak  shown  hi  Fig.  Boa,  the  lateral  pits  are  arranged  in  an 
alternating  fashion.  Again,  in  b,  representing  the  lateral  pitting 
of  the  vessel  in  the  poplar,  alternation  is  accompanied  by  crowding 
to  such  an  extent  that  the  pits  become  angular  by  mutual  contact. 


FIG.  80. — Lateral  pitting  of  vessels  of  dicotyledons.     Explanation  in  the  text 

In  c  is  shown  the  characteristic  pitting  of  the  vessels  of  the  vine 
family  (Vitaceae).  Here  the  sculpture  of  the  lateral  walls  consists 
of  elongated  slits  closed  by  equally  elongated  membranes.  A 
reference  to  the  structure  of  the  secondary  wood  in  the  region  of 
the  pith  in  this  instance  makes  it  clear  that  the  slitlike  pits  of  the 
Vitaceae  are  not  persistent  scalariform  pores  of  the  first-formed 
region  of  the  xylem,  but  are  the  result  of  the  lateral  fusion  of 
horizontal  rows  of  pits  in  the  side  walls  of  the  vessels.  In  d  is 
represented  a  condition  of  structure  of  the  vascular  wall  which  is 
common  in  the  higher  types  of  dicotyledonous  woods.  Here  the 
alternating  pitting  is  overlaid  internally  by  spiral  structures  which 
it  is  customary  to  call  tertiary  thickenings. 

In  concluding  the  statements  in  regard  to  vessels  it  is  necessary 
to  refer  to  the  phenomenon  of  tylosis  or  occlusion  of  the  lumina 
or  cavities  of  the  vessels  by  parenchymatous  ingrowths.  These 


104 


THE  ANATOMY  OF  WOODY  PLANTS 


structures  originate  under  various  conditions,  but  are  more  usually 
found  in  connection  with  those  changes  which  precede  the  trans- 
formation of  sapwood,  or  alburnum,  into  heartwood,  or  duramen. 
As  the  living  cells  in  proximity  to  the  vessels  (whether  they  belong 
to  the  radial  or  vertical  systems  of  storage  tissues  appears  not  to 


FIG.  81. — Tyloses  in  oak  and  locust.     Explanation  in  the  text 


be  of  importance)  are  about  to  die,  they  push  processes  into  the 
adjacent  vessels  by  the  bulging  and  growth  of  the  cellulose  mem- 
branes of  the  pits  connecting  them  with  the  vascular  structures. 
These  ingrowths  are  occupied  by  protoplasm  and,  sometimes  at 
any  rate,  by  nuclei  as  well.  When  abundantly  developed,  they 
completely  close  the  lumen  of  the  vessel  and  render  impossible  the 
passage  of  water.  Fig.  43,  on  page  57,  has  made  clear  the  contrast 
resulting  from  this  phenomenon  in  the  sapwood  and  the  heartwood 


FIBRO  VASCULAR  TISSUES:  VESSELS 


of  the  oak.  In  Figs.  8ia  and  b  is  shown  the  phenomenon  of  tylosis 
in  the  vessels  of  the  white  oak.  In  this  species  all  the  larger  vessels 
are  more  or  less  completely  occluded 
by  tyloses  extending  from  and  only 
excluding  the  latest  complete  annual 
ring.  In  c  and  d  a  similar  condition  is 
represented  in  the  case  of  Quercus 
Engelmanni.  In  this  instance  the 
tyloses  have  developed  a  very  thick  and 
strongly  pitted  wall,  a  condition  more 
rarely  found.  In  e  and  /  tylosis  as 
found  in  the  case  of  the  black  locust, 
Robinia  Pseudacacia,  is  represented. 
Here  the  ingrowths  derived  from  adjoin- 
ing parenchyma  cells  are  very  numer- 
ous and  form  a  mass  completely 
occluding  the  cavity  of  the  vessel.  In 
Fig.  82  are  seen,  both  in  transverse 
view,  earlier  stages  of  the  invasion  of  a 
vessel  in  the  oak  through  the  agency  of 
tyloses.  The  representation  in  this 


FIG.  82. — Diagrammatic 
representation  of  the  process 
of  tylosis. 


illustration  is  somewhat  diagram- 
matic to  show  more  clearly  the 
entry  of   the  ingrowths  through 
the  medium  of  the  pits  in   the 
walls  of  the  vessels. 
The  phenomenon  of  tylosis  is  in  all  probability  an  extremely 
ancient  one  for  the  higher  plants,  as  it  is  found  far  back  into  the 


FIG.  83. — Tylosis  in  tracheids  of 

Liquidambar . 


io6  THE  ANATOMY  OF  WOODY  PLANTS 

Mesozoic.  It  is  not  confined  by  any  means  to  vessels  and  even 
occurs  rarely  in  the  tracheids  of  dicotyledons.  Fig.  83  shows  the 
presence  of  tylosis  in  the  fiber-tracheids  of  the  sweet  gum  (Liquid- 
ambar  styraciflua) .  The  heavily  outlined  bodies  in  the  cavities  of 
certain  of  the  fibrous  elements  are  of  the  nature  of  tyloses.  Similar 
conditions  have  been  found  in  the  case  of  other  dicotyledonous 
woods.  It  has  been  made  clear  in  an  earlier  chapter  that  ingrowths 
resembling  tyloses  occur  in  the  resin  canals  of  the  pine  in  the  region 
of  the  heartwood.  Occlusion  by  means  of  parenchymatous  inva- 
sions is,  however,  not  confined  to  the  resiniferous  spaces  in  the 
genus  mentioned.  In  the  roots  and  likewise  in  the  cone  the  tra- 
cheids of  the  wood  often  show  themselves  occluded  by  ingrowing 
parenchyma  cells.  Such  conditions  are  not  normally  found  in  the 
vegetative  stem  of  living  pines,  although  they  are  known  to  occur 
in  the  branches  of  cretaceous  Pityoxyla  from  the  Eastern  United 
States. 


CHAPTER  VIII 
THE  FIBROVASCULAR  TISSUES— PHLOEM 

The  ligneous  or  woody  tissues  which  have  been  discussed  under 
various  headings  in  previous  chapters  are  of  great  importance  on 
account  of  their  conservatism.  They  manifest  a  high  degree  of 
differentiation  and  are  also  important  by  reason  of  the  fact  that 
their  relative  imperishability  has  resulted  in  their  being  abundantly 
preserved  as  fossils  from  the  most  ancient  times  to  the  present. 
Woods  consequently  furnish  on  the  whole  the  most  important 
historical  document  in  favor  of  the  hypothesis  of  evolution,  and 
those  who  are  interested  in  the  doctrine  of  descent  as  applied  to 
the  case  of  plants  cannot  afford  to  neglect  the  investigation  of 
the  ligneous  tissues  of  the  vascular  plants.  The  situation  for  the 
plants  provided  with  tracheary  tissues  is,  in  fact,  very  different 
indeed  from  that  presented  by  the  lower  vegetable  organisms,  where 
evolutionary  data  are  necessarily  largely  speculative  and  experi- 
mental. Invariably  associated  with  the  woody  tissues  are  those  of 
the  phloem.  The  primal  function  of  the  woody  tissues  was  appar- 
ently to  conduct  water  into  the  superior  or  outlying  parts  of  the 
plant.  To  this  function  have  been  superadded  in  the  long  course 
of  geologic  time,  as  pointed  out  in  preceding  chapters,  the  functions 
of  strength  and  storage,  with  corresponding  and  appropriate 
modifications  of  structure.  Just  as  the  wood  in  the  first  instance 
served  the  purpose  of  the  movement  of  water  from  the  soil,  so  the 
primal  condition  of  the  tissues  of  the  phloem  was  that  of  conducting 
the  elaborated  foodstuffs  manufactured  in  the  leaves  to  other 
parts  of  the  plant  where  they  are  either  utilized  in  the  processes 
of  growth  and  respiration  or  stored  up  as  reserves. 

The  primitive  type  of  phloem,  as  that  of  xylem,  appears  to  have 
existed  in  the  case  of  the  lepidodendrids.  Its  structure  has  not 
been  satisfactorily  preserved  in  these  primitive  vascular  plants,  and 
there  is  some  difference  of  opinion  as  to  interpretation.  Some 
investigators  have  expressed  the  opinion  that  no  true  phloem  is 

107 


io8  THE  ANATOMY  OF  WOODY  PLANTS 

present  in  these  ancient  organisms,  while  others  have  contended 
for  a  very  simple  type  of  organization  in  the  arboreal  club  mosses 
of  the  Paleozoic  age.  In  view  of  the  differences  of  opinion  and  the 
uncertainty  of  the  data  in  the  case  of  the  phloem  of  Paleozoic 
Pteridophyta,  it  will  be  well  to  confine  our  attention  in  this 
instance  to  the  living  representatives  of  the  fern  alliance. 

Fig.  84  shows  the  detailed  organization  of  one  of  the  fibro- 
vascular  strands  of  the  bracken  Pteris  aquilina.     The  bundle  as  a 


FIG.  84.— Bundle  of  Pteris  aquilina.    Explanation  in  the  text 

whole  is  clearly  limited  by  a  dark  uniseriate  layer,  the  endodermis, 
which  is  ordinarily  interpreted  as  the  innermost  layer  of  the  funda- 
mental tissues  in  juxtaposition  to  the  fibrovascular  system.  Within 
the  endodermis  is  situated  a  layer  one  or  sometimes  two  cells  in 
breadth,  the  pericycle,  which  constitutes  the  external  boundary 
of  the  fibrovascular  tissues,  just  as  the  endodermis  marks  the 
internal  limit  of  the  fundamental  system.  Next  to  the  pericycle 
are  certain  extremely  minute,  apparently  empty,  cells,  the  first- 
formed  elements  of  the  phloem,  or,  as  they  are  technically  desig- 
nated, the  protophloem.  Next  inward  lie  storage  cells,  nucleate 


FIBROVASCULAR  TISSUES:  PHLOEM  109 

and  more  or  less  loaded  with  starch.  These  are  the  parenchyma 
of  the  phloem.  We  are  not  acquainted  with  the  origin  of  paren- 
chymatous  cells  in  the  phloem  for  reasons  indicated  at  the  outset 
of  the  present  chapter.  Farther  inward  lie  the  larger  conductive 
elements  of  the  phloem,  known  as  the  metaphloem.  These  are 
cells  which  have  no  contents  but  a  delicate  lining  of  protoplasm, 
and  this  does  not  appear  in  the  illustration.  The  elements  under 
discussion  as  well  as  those  of  the  protophloem  are  known,  on 
account  of  certain  structural  features  to  be  described  later,  as 
sieve  tubes.  It  is  important  to  note  in  the  present  connection  that 
sieve  tissues  do  not  present  the  variety  of  modes  of  development 
which  characterizes  the  xylem.  Almost  invariably  in  this  cate- 
gory of  tissues  the  first-formed  elements  are  external  and  those  of 
later  appearance  are  laid  down  in  centripetal  order.  As  a  con- 
sequence the  terms  exarch,  mesarch,  and  endarch,  which  are  so  sig- 
nificant in  connection  with  the  development  of  the  primary  wood, 
have  little  bearing  in  the  case  of  the  sieve  tissues.  Internal  to  the 
larger  sieve  elements  of  the  metaphloem  lies  a  more  or  less  continu- 
ous band  of  parenchyma,  separating  the  phloem  from  the  xylem. 
The  tracheary  elements  of  the  wood  are  individually  more  or  less 
completely  surrounded  with  parenchyma.  The  phloem  does  not 
form  a  continuous  band  about  the  xylem,  but  is  interrupted  at  the 
two  ends  of  the  elongated  mass  of  wood.  In  certain  ferns  the  tissues 
of  the  phloem  occur  only  on  one  side  of  the  bundle,  in  which  case  the 
strand  of  fibrovascular  tissue  is  known  as  collateral.  Where  the 
phloem  forms  a  complete  jacket  about  the  xylem  the  condition 
is  known  as  concentric;  and  where,  as  in  the  bundle  appearing  in 
the  figure  under  discussion,  the  sieve  tissues  are  confined  to  two 
opposite  sides  the  condition  may  be  designated  as  bicollateral. 

In  Fig.  85  is  represented  a  longitudinal  view  of  part  of  the 
same  bundle.  The  sieve  tubes  now  appear  as  elongated  cells 
with  tapering  ends  and  sculptured  walls.  The  relief  of  the  walls 
consists  of  somewhat  angular  crowded  areas,  although  on  the 
whole  they  are  tolerably  evenly  distributed.  These  areas  are  per- 
forated with  very  small  simple  pits  and  are  the  sieve  plates.  It  is 
these  structures  which  give  to  the  sieve  tube  its  name.  After  the 
characteristic  elements  of  the  phloem  have  reached  a  certain  age 


no 


THE  ANATOMY  OF  WOODY  PLANTS 


their  pores  become  more  or  less  completely  occluded  by  slimy  plugs 
known  as  callus.  The  details  of  origin  and  the  significance  of  this 
substance  are  not  fully  understood  and  from  the  standpoint  of 
the  present  statement  are  of  relatively  little  importance. 

In  Fig.  86  is  re- 
produced a  photo- 
graph of  the  wood 
and  inner  bark  of  the 
pine.  The  xylem  is 
clearly  very  regular 
in  its  structure  and 
consists  of  a  series  of 
annual  rings,  the 
organization  of  which 
has  been  dealt  with 
in  a  preceding  chap- 
ter. In  contrast  to 
the  persisting  regu- 
larity of  the  wood  the 
phloem  presents  itself 
as  a  mass  of  tissue 
radially  seriate  only 
for  a  short  distance 
outward  from  the 
cambium,  or  zone  of 
growth.  Externally 
its  elements  a  re 
thrown  into  more  or 
less  meandering  lines 
as  a  result  of  the  col- 
lapse of  certain  of  its  constituent  cells,  which  will  be  considered  more 
in  detail  in  the  sequel.  The  section  reproduced  in  the  photograph 
under  discussion  was  made  from  material  secured  in  the  winter 
period  of  rest.  The  uncollapsed  part  of  the  phloem  corresponds 
to  a  single  year's  growth,  and  all  that  region  of  the  inner  bark  or 
phloem  characterized  by  the  meandering  course  of  the  rays  is  non- 
functional so  far  as  the  most  characteristic  elements,  the  sieve 


FIG.  85. — Longitudinal  view  of  part  of  a  bundle 
of  Pteris  aquilina.  On  the  left  is  shown  a  vessel, 
while  toward  the  right  and  represented  in  black  are 
sieve  tubes. 


FIBRO VASCULAR  TISSUES:  PHLOEM 


ill 


tubes,  are  concerned.  Fig.  87  shows  a  highly  magnified  view  of  the 
region  immediately  outside  and  inside  the  zone  of  growth,  or  cam- 
bium. Internal  to  the  active  layer  the  wood  is  recognizable  by  its 
bordered  pits.  The  initial  region,  known  as  the  cambium,  is  dis- 
tinguishable by  the  richly  protoplasmic  character  of  its  cells  and 
their  thin  tangential  walls,  which  are  in  contrast  with  the  thick- 
ness of  the  radial 
boundaries.  Ex- 
ternal to  the  cam- 
bium  lies  the 
phloem,  composed 
most  characteristi- 
cally of  somewhat 
rectangular  cells, 
the  sieve  tubes.  In 
these,  unlike  the 
cambial  elements, 
there  is  no  nucleus, 
and  the  scanty 
parietal  proto- 
plasm presents 
likewise  a  striking 
difference  from  the 
richly  protoplas- 
mic cambial  region. 

Scattered  thinly  through  the  phloem  are  a  few  rounded  cells 
which  contain  protoplasm  and  grains  of  starch  (represented 
black  as  if  stained  with  iodine).  The  rays  cross  the  longitudinal 
elements  of  the  phloem,  the  sieve  tubes,  and  the  parenchyma  cells 
at  right  angles  and  appear  in  the  figure  as  single  files  running  nearly 
straight  for  a  short  distance  and  then  pursuing  a  meandering  course 
outward.  The  cells  of  the  two  rays  shown  in  the  figure  present 
a  different  appearance.  In  the  file  of  radial  parenchyma  on  the 
right  the  elements  are  filled  with  protoplasm  and  show  the  presence 
of  a  nucleus.  In  addition,  a  considerable  amount  of  starch  is 
present  in  the  form  of  grains  tinctured  black  by  iodine.  It  is  clear 
that  the  cellular  elements  of  the  ray  to  the  right  continue  elements 


FIG.  86. — Inner  bark  with  cambium  and  adjacent 
wood  of  the  pine. 


112 


THE  ANATOMY  OF  WOODY  PLANTS 


FIG.  87. — Transverse  section  of  xylem,  phloem,  and  cambium  of  the  pine,  more 
highly  magnified.    Explanation  in  the  text. 


FIBROVASCULAR  TISSUES:  PHLOEM  113 

of  a  similar  nature  in  the  portion  of  the  ray  contained  in  the  wood. 
If  we  now  turn  our  attention  to  the  ray  on  the  left,  a  somewhat 
different  situation  is  presented  to  the  eye.  Here  the  cells  are 
entirely  without  starch,  although  they  still  show  protoplasm  and  a 
nucleus.  Internally  they  are  in  line  with  the  marginal  tracheids 
of  the  ray  in  the  wood,  which  have  been  described  in  an  earlier 
chapter.  Obviously  there  are  two  categories  of  cells  in  the  rays 
of  the  phloem  in  Pinus,  just  as  has  been  shown  to  be  the  case 
for  the  radial  parenchyma  of  the  xylem.  It  will  now  be  con- 
venient to  consider  the  relations  of  the  various  elements  of  the 
phloem  to  one  another.  In  this  connection  we  may  first  discuss 
the  sieve  tubes.  It  will  be  noticed  that  there  are  certain  dark  lines 
stained  with  iodine  occurring  in  groups  on  the  radial  walls  of  the 
sieve  elements.  These  are  the  transverse  sections  of  sieve  pores. 
Parenchymatous  cells  of  the  phloem  have  no  intimate  relation  to 
the  tubes,  as  no  pits  facilitate  interchanges  between  the  two  types 
of  elements.  The  same  statement  holds  hi  regard  to  the  cells  of  the 
ray  shown  on  the  right  side  of  the  figure.  Orr  the  left,  however,  the 
elements  of  the  ray  are  clearly  related  to  the  sieve  tubes  by  means 
of  sieve  plates,  distinctly  differentiated  on  the  side  of  the  sieve 
tubes,  but  not  well  developed  on  the  side  of  the  ray.  It  is  evident 
that  there  is  a  specially  intimate  connection  between  certain  ele- 
ments of  the  ray,  which  are  contrasted  with  the  remaining  constitu- 
ents by  the  absence  of  starch,  and  the  conductive  elements  of  the 
phloem  or  sieve  tubes. 

If  the  organization  or,  more  correctly,  the  disorganization  of  the 
phloem  is  followed  more  externally,  it  will  be  observed  in  the  illustra- 
tion that  the  ray  on  the  right  continues  to  retain  its  protoplasmic 
contents  and  its  grains  of  starch,  while  that  on  the  left  has  entirely 
lost  its  living  contents.  A  further  feature  of  the  phloem  as  repre- 
sented in  the  external  region  of  the  figure  is  the  distortion  and  final 
collapse  of  the  sieve  tubes,  preceded  by  the  loss  of  the  delicate  pro- 
toplasmic lining  surrounding  the  inner  walls  of  these  elements.  The 
parenchymatous  cells  retain  their  integrity  in  the  collapsed  region  of 
the  phloem,  precisely  as  is  the  case  with  the  ray  indicated  on  the 
right  of  the  figure.  Another  feature  of  the  disorganization  of  the 
phloem  is  the  appearance  of  large  masses  or  plugs  of  material, 


II4  THE  ANATOMY  OF  WOODY  PLANTS 

appearing  black  in  the  figure,  which  are  present  exclusively  on  the 
radial  walls  of  the  elements,  although  in  some  instances  the  col- 
lapsed state  of  the  tubes  gives  rise  to  the  appearance  of  a  tangential 
position  for  the  bodies  under  discussion.  The  plugs  mentioned 
above  constitute  the  callus,  a  substance  which  in  the  conifers 
and  the  great  majority  of  seed  plants  higher  in  the  scale  makes 
its  appearance  on  the  sieve  plate  at  the  time  the  tube  is  in  the 
initial  stages  of  collapse.  The  callus  completely  blocks  the  pores 
of  the  sieve  plate  and,  after  persisting  for  a  short  time,  disappears, 
leaving  the  pores  of  the  sieve  area  quite  open.  It  is  also  apparent 
that  in  the  case  of  the  ray  cells  on  the  left  of  the  figure  the  callus 
present  is  unilateral  and  occurs  only  on  the  side  of  the  sieve  tube. 
Obviously  the  type  of  ray  element  appearing  on  the  left  in  the 
illustration  is  definitely  related  in  its  duration  to  the  life  of 
the  sieve  tube,  and  both  structures  cease  to  be  functional  at  the 
same  time. 

In  the  next  figure  (Fig.  88)  the  radial  aspect  of  the  phloem  is 
shown.  Here  the  ray  naturally  appears  in  longitudinal  section. 
Above  and  below  in  the  region  of  the  phloem  it  shows  vertically 
placed  cells  which  contrast  in  the  position  of  their  axis  of  greatest 
length  to  the  central  elements  of  the  radial  parenchyma.  These  so- 
called  "erect  cells"  correspond  to  the  elements  shown  in  the  ray 
to  the  left  of  Fig.  87  and  are  similarly  characterized  by  the  absence 
of  the  starch,  which  is  a  feature  of  the  contents  of  the  central  or 
prone  cells  of  the  ray.  In  passing  toward  the  right  from  the  cam- 
bium the  protoplasmic  filling  of  these  elements  becomes  less 
abundant,  until  finally,  where  the  callus  presents  itself  in  face 
view,  it  disappears  altogether,  in  this  respect  presenting  a  con- 
trast to  the  central  cells  which  retain  their  protoplasm  and  starch 
indefinitely.  The  sieve  tubes  now  appear  in  longitudinal  view, 
and  nearer  the  cambium  they  have  a  delicate  protoplasmic  lining 
often  containing  some  small  grains  of  so-called  transitory  starch. 
The  sieve  areas,  or  sieve  plates,  are  presented  to  the  observer  in 
face  view,  and  toward  the  extreme  right  callus  plugs  occlude  their 
pores  and  at  the  same  time  the  tube  has  lost  its  living  contents. 
.It  is  clear  in  the  radial  view,  as  in  the  transverse,  that  sieve  tube 
and  marginal  erect  cell  cease  to  exist  and  develop  callus  plugs 


FIBROVASCULAR  TISSUES:  PHLOEM  115 

simultaneously.  A  single  row  of  parenchymatous  elements  is 
represented  as  crossing  the  axis  of  the  ray.  To  the  left  of  the 
cambium  lies  the  wood  with  its  tracheids  and  the  continuation  of 
the  ray.  The  radial  structures  in  the  xylem  show  also  a  differentia- 
tion into  central  and  marginal  cells,  but  here  ray-tracheids  take 
the  place  of  the  erect  cells  of  the  ray  in  the  phloem. 


FIG.  88. — Radial  section  through  xylem,  cambium,  and  phloem  of  the  pine. 
Explanation  in  the  text. 

In  Fig.  89  appears  the  tangential  section  of  the  tissues  of  the 
phloem  in  Pinus.  The  plane  of  incidence  of  the  section  is  tan- 
gentially  slightly  oblique;  hence  elements  of  different  ages  appear 
on  the  opposite  sides.  Toward  the  right  are  represented  sieve 
tubes  in  which  the  protoplasmic  lining  has  disappeared  and  the 
sieve  areas  (seen  here  in  profile)  are  blocked  by  masses  of  callus. 
In  the  case  of  the  rays  which  are  naturally  shown  in  transverse 
section  the  marginal  or  erect  elements  are  calloused  on  the  side  of 
the  tubes.  The  central  cells  of  the  rays  still  show  the  presence  of 


n6 


THE  ANATOMY  OF  WOODY  PLANTS 


protoplasm  and  starch.  In  the  middle  region  of  the  figure  a  file 
of  parenchyma  cells  is  present.  To  the  left  of  the  middle  line  the 
sieve  elements  show  the  delicate  protoplasmic  lining  which  char- 
acterizes them  in  the  functional  condition.  The  radial  parenchyma 
is  also  entirely  in  a  living  condition,  although  the  marginal  cells 
are  differentiated  by  the  absence  of  the  starch  contents  found  in  the 


FIG.  89. — Tangential  section  through  the  phloem.     Explanation  in  the  text 

central  region  of  the  ray  structure.     The  callus  in  both  sieve  tubes 
and  marginal  cells  is  here  conspicuous  by  its  absence. 

A  somewhat  complicated  condition  of  the  phloem  has  purposely 
been  chosen  in  the  case  of  the  coniferous  gymnosperms,  because 
there  is  good  reason  to  believe  that  in  this  group,  which  is  a  decadent 
one,  the  more  elaborately  organized  condition  is  antecedent  to  that 
marked  by  a  greater  degree  of  simplicity.  On  the  whole,  the  pine 
and  its  allies  represent  the  most  highly  differentiated  structure  of  the 
phloem  in  the  group.  In  the  Cupressineae,  Taxodineae,  Araucari- 
neae,  Podocarpineae,  and  Taxineae  the  radial  structures  of  the 
phloem  show  a  marked  degree  of  simplification  as  compared  with 


FIBRO VASCULAR  TISSUES:   PHLOEM 


117 


the  pine  family  proper.  It  is  not  necessary  in  an  elementary  work 
like  the  present  one  to  enlarge  further  on  the  organization  of  the 
phloem  of  the  gymnosperms  in  general  or  the  conifers  in  particular. 
The  structure  of  the  phloem  in  the  angiosperms,  and  especially  of 
arboreal  dicotyledons,  will  next  occupy  our  attention.  Here  the 


FIG.  90. — Transverse  section  of  the  xylem,  cambium,  and  phloem  in  the  American 
linden.     Explanation  in  the  text. 

general  organization  is  quite  different  from  that  characteristic  of 
gymnospermous  groups  and  accordingly  merits  detailed  considera- 
tion. Fig.  90  shows  a  transverse  section  of  the  region  immediately 
internal  and  external  to  the  cambium  in  the  trunk  of  Tilia  americana, 
the  American  linden  or  basswood.  Toward  the  lower  side  of  the 


n8  THE  ANATOMY  OF  WOODY  PLANTS 

illustration  appears  the  xylem,  consisting  of  vessels,  libriform 
fibers,  parenchyma,  and  wood  rays.  Above  the  tissue  so  organized 
lies  the  cambium,  or  zone  of  growth,  which  alternately  adds  ele- 
ments to  the  structure  of  the  xylem  and  that  of  the  phloem.  The 
latter  tissue  appears  above  the  cambium  in  the  figure  and  outside 
it  in  the  trunk  of  the  tree.  The  preparation  which  served  as  the 
basis  of  the  illustration  was  made  from  material  secured  during  the 
period  of  winter  rest.  Next  to  the  cambium  lie  fibrous  and  similar 
elements,  to  the  abundant  development  of  which  in  its  inner  bark 
the  basswood  (literally  bastwood  or  tree  useful  for  binding)  owes 
its  name.  The  fibers  of  the  phloem  are  often  called  hard  bast. 
Examination  of  the  figure  will  show  that  the  fibrous  elements  are 
separated  from  the  cambium  by  a  row  of  cells  either  quite  empty 
or  containing  large  crystals  of  calcium  oxalate.  These  are  the  so- 
called  crystallogenous  cells  and  are  very  commonly  present  inter- 
nal to  the  zones  of  fibers  or  hard  bast  in  the  phloem  of  the  form 
•under  discussion.  External  to  the  hard  bast  lies  a  zone  distin- 
guished by  the  presence  of  cells  either  richly  protoplasmic  as  to  their 
contents  or,  in  case  the  protoplasm^  is  more  scanty,  characterized  by 
relatively  thin  and  unlignified  walls.  This  region  is  the  so-called 
soft  bast  and  shows  considerable  complexity  of  organization. 
First  taking  the  cells  with  abundant  protoplasm,  we  see  clearly  that 
these  can  be  divided  into  two  categories — namely,  those  which 
contain  grains  of  starch  (represented  in  black  as  if  treated  with 
strong  iodine  solution)  and  those  in  which  amylaceous  substances 
are  absent.  The  latter  are  further  distinguished  by  their  generally 
small  size  and  somewhat  triangular  shape,  due  to  the  fact  that  they 
are  usually  accommodated  in  the  angles  of  large  thin-walled  ele- 
ments with  a  delicate  protoplasmic  lining.  The  triangular  cells 
are  known  as  "companion  cells"  and  are  a  constant  feature  of 
structure  of  phloem  of  the  angiosperms  as  contrasted  with  the 
gymnosperms.  The  larger  elements  of  the  phloem  with  thin  walls 
and  scanty  parietal  protoplasm  are  the  sieve  tubes.  In  some  in- 
stances the  sieve  plate  can  be  seen  in  the  transverse  section  repro- 
duced in  the  figure,  and  it  is  clear  that  its  position  is  characteristically 
radial.  Farther  outward  the  densely  filled  parenchymatous  cells 
of  the  soft  bast  give  place  again  to  the  dead  crystallogenous  elements, 


FIB RO VASCULAR  TISSUES:  PHLOEM 


119 


which  are  in  turn  followed  by  bast  fibers.  The  rays  of  the  phloem 
in  the  basswood  and  other  dicotyledons  show  no  special  feature  of 
interest,  since  they  do  not  in  any  case  manifest  the  intimate  rela- 
tion to  the  sieve  tubes  described  above  in  the  pine. 


FIG.  91. — Radial  section  of  the  xylem,  cambium,  and  phloem  of  the  linden,  of 
basswood.  Explanation  in  the  text. 

In  the  next  illustration  (Fig.  91)  the  radial  aspect  of  the  phloem 
in  Tilia  is  shown.  Here  the  rays  appear  naturally  in  longitudinal 
section  and  are  composed  of  heavily  pitted  cells  containing  dense 
protoplasm  and  much  starch.  Above  the  ray  are  represented  the 
various  elements  of  the  hard  and  soft  bast.  On  the  extreme  left 


120  THE  ANATOMY  OF  WOODY  PLANTS 

are  to  be  seen  the  summer  elements  of  the  wood  in  contact  with 
the  cambium.  They  are  followed  by  parenchyma  and  by  crystal- 
logenous  cells,  empty  and  heavily  pitted  on  their  horizontal  walls. 
Next,  the  fibers  of  hard  bast  meet  the  view,  and  these  pass  in  suc- 
cession into  bast  parenchyma,  characterized  by  heavy  pitting, 
abundant  starch,  and  protoplasmic  contents,  and  by  sieve  tubes  of 
large  lumen  and  scanty  parietal  protoplasm.  Within,  in  the  walls 
of  the  sieve  elements,  lie  the  companion  cells,  distinguished  by  their 
slender  form  and  the  absence  of  starch  in  their  protoplasm.  The 
sieve  tubes  in  the  radial  aspect  present  to  the  eye  strongly  inclined 
end  walls  which  may  be  compared  with  the  similarly  slanting 
partitions  in  the  vessels  of  the  wood.  The  sieve  element  is,  in 
fact,  the  exact  analogue  of  the  vessel,  and,  just  as  the  vascular 
elements  in  angiospermous  woods  play  the  main  part  in  the  trans- 
port of  water,  so  the  sieve  tubes  perform  the  principal  role  in  the 
movement  of  the  elaborated  organic  stuffs  from  the  leaves.  Another 
feature  of  analogy  between  the  sieve  tube  and  the  vessel  is  not 
only  the  more  or  less  inclined  terminal  walls  at  angles  to  the  lateral 
ones,  but  also  the  function  of  transport  specially  provided  for  in 
connection  with  the  ends  of  the  element. 

In  the  vessel  this  condition  finds  its  expression  in  the  develop- 
ment of  scalariform  or  porous  perforations  and  in  the  case  of  the 
sieve  tubes  by  the  appearance  of  particularly  extensive  and  large- 
pored  sieve  plates  on  the  terminal  inclined  walls;  and  these  in 
highly  specialized  forms,  such  as  herbaceous  dicotyledons  and 
monocotyledons,  may  be  the  only  functional  regions.  Following 
the  sieve  tubes  is  a  zone  of  bast  parenchyma,  succeeded  in  turn  by 
another  zone  of  crystall'ogenous  cells  and  hard-bast  fibers. 

In  Fig.  92  is  shown  the  tangential  longitudinal  aspect  of  the 
phloem  in  the  basswood.  Here  the  rays  naturally  appear  in  trans- 
verse section  and  are  clearly  of  diverse  sizes,  but  are  all  composed 
of  strongly  pitted  cells  containing  abundance  of  protoplasm  and 
starch.  In  this  aspect  the  plane  of  section  is  purposely  slightly 
oblique  so  as  to  exhibit  all  categories  of  constituents  of  the  phloem 
at  the  same  time.  To  the  left  lie  crystallogenous  cells  near  a  large 
ray.  The  ray  is  in  contact  on  the  opposite  side  with  parenchyma  of 
the  soft  bast.  Then  follow  sieve  tubes  and  their  related  com- 


FIBROVASCULAR  TISSUES:    PHLOEM 


121 


panion  cells.  In  the  tangential  aspect  the  terminal  sieve  plate, 
since  it  is  radial  in  position,  is  shown  in  profile.  The  protoplasm, 
present  more  densely  in  jthe  region  of  the  terminal  plates,  is  pur- 
posely omitted  in  ordpr'ihat  the  sculpture  of  the  lateral  walls  may 


Ir 


FIG.  02. — Tangential  section  of  the  phloem  of  the  linden,  c  Explanation  in  the 
text. 

>     ^A>  ^ 

be  apparent.  There  are  manifestly  strongly  punctuated  areas  in 
the  side  walls  of  the  sieve  element.  These  represent  sieve  plates 
which  have  more  or  less  completely  lost  their  function  as  a  result 
of  degeneracy.  In  herbaceous  types  lateral  sieve  areas  frequently 
have  no  longer  even  a  morphological  expression,  the  structural 


122 


THE  ANATOMY  OF  WOODY  PLANTS 


as  well  as  the  functional  plates  being  almost  exclusively  limited 
to  the  terminal  walls.  Abortive  and  lateral  sieve  plates  in  the 
angiosperms  are  known  as  lattices.  In  woody  dicotyledons  the 
lattices  are,  in  the  case  of  lower  types,  often  well  developed  and 
are  likewise  frequently  functional  as  actual  sieve  plates.  Farther 


FIG.  93. — Transverse  section  of  the  phloem  in  the  grapevine.    Explanation  in 
the  text. 


to  the  right  the  figure  shows  more  parenchyma  and  additional 
fibers  of  the  hard  bast. 

For  comparison  with  the  basswood  is  presented  in  Fig.  93  the 
transverse  view  of  the  phloem  in  the  grapevine  (Vitis  species). 
To  the  right  lies  a  large  ray  of  the  compound  type.  To  the  left 
of  this  is  situated  the  phloem  proper,  consisting,  as  is  often  the 
case  in  the  dicotyledons,  of  hard  and  soft  bast.  The  former  tissue  is 
composed  of  fibers  which  differ  chiefly  from  the  corresponding 
structures  in  Tilia  by  the  fact  that  they  are  septate  when  viewed  in 


FIBROVASCULAR  TISSUES:    PHLOEM  123 

longitudinal  section.  The  soft  bast  consists  of  parenchyma,  sieve 
tubes,  and  their  companion  cells.  Crystallogenous  elements  are 
inconspicuous  in  the  bast  of  the  grapevine.  The  sieve  tubes  in 
some  cases  show  their  terminal  and  mainly  functional  sieve 
areas  in  transverse  section.  The  plate  in  this  instance  is  blocked 
by  a  formation  of  callus  which  in  the  vine  covers  the  plate  each 
autumn  and  disappears  again  in  the  following  spring  when 
vegetative  activity  is  renewed.  This  condition  is  unusual,  since  . 
quite  generally  in  the  angiosperms,  as  in  the  gymnosperms,  the 
appearance  of  a  callus  marks  the  end  of  the  life  of  the  sieve 
tube.  A  seasonal  callus  is  likewise  found  in  the  barberry  and  in 
certain  other  instances.  The  final  callus,  marking  the  dissolution 
of  the  characteristic  functional  elements  of  the  phloem,  is  known  as 
the  definitive  callus.  The  tubes  of  the  phloem  in  the  grapevine 
are  of  interest  in  showing  a  considerable  amount  of  so-called  transi- 
tory starch,  indicated  in  the  figure  by  the  larger  black  granules. 

Both  the  xylem  and  the  phloem  of  herbaceous  dicotyledons  and 
of  the  monocotyledons  show  interesting  modifications  of  the  con- 
ditions characteristic  of  arboreal  and  perennial  forms.  Here  the 
vessel  and  the  sieve  tube  constitute  to  an  increasing  extent,  and 
sometimes  exclusively,  the  functional  elements  of  the  two  important 
regions  of  the  fibrovascular  bundle  and  are  present  in  what  may  be 
regarded  with  a  strong  degree  of  probability  as  their  highest  and 
most  specialized  form.  The  vessel  is  practically  always  porous  in 
herbs  and  presents  as  a  usual  peculiarity  of  organization  terminal 
walls  which  are  nearly  or  quite  horizontal.  The  sieve  tube  also 
tends  toward  horizontal  end  walls,  and  the  sieve  plates  have  prac- 
tically disappeared  from  the  lateral  walls,  the  terminal  areas  alone 
being  functionally  important. 

In  concluding  the  statement  regarding  the  organization  of  the 
structures  in  the  phloem  it  will  be  well  to  represent  side  by  side 
the  stereoscopic  aspects  of  the  several  types  of  sieve  tubes  in  com- 
parison with  corresponding  varieties  presented  by  tracheids  and 
vessels.  In  Fig.  940  is  shown  diagrammatically  the  organization  of 
a  sieve  tube  in  the  secondary  phloem  of  a  gymnosperm.  It  is  clear 
that  the  plates  are  confined  to  two  opposite  walls,  which  are  the 
radial  ones.  The  cell,  further,  tapers  imperceptibly  to  a  point  at 


124 


THE  ANATOMY  OF  WOODY  PLANTS 


-^f^Z&t 

f 


FIG.  94. — Diagram  showing  types  of  sieve  tubes  and  vessels  in  the  higher  vascular 
plants.    Explanation  in  the  text. 


FIBROVASCULAR  TISSUES:   PHLOEM  125 

either  end.  For  comparison  with  this  condition  is  shown  in  b  the 
sieve  element  of  an  arboreal  dicotyledon.  Here  the  terminal  walls 
are  distinctly  at  angles  with  the  lateral  ones  and  bear  the  best- 
developed  sieve  plates.  Laterally,  on  two  opposite  and  usually 
tangential  sides,  are  situated  the  degenerate  sieve  plates,  or  lattices. 
The  third  item,  c,  visualizes  the  type  of  sieve  tube  characteristic 
of  an  advanced  herbaceous  dicotyledon  or  of  a  monocotyledon.  In 
this  instance  the  terminal  wall  of  the  sieve  tube  is  horizontal,  and 
parietal  lattices  are  conspicuous  by  their  absence.  This  state 
represents  the  highest  degree  of  differentiation  of  the  sieve  element. 
In  the  following  items  are  delineated  corresponding  conditions  for 
the  water-conducting  elements  of  the  xylem.  In  d  appears  a  tra- 
cheid  from  the  spring  wood  of  Pinus,  showing  tapering  or  fusiform 
configuration  and  exclusively  radial  pitting.  Next,  in  e,  appear 
the  main  vessel  types  of  Gnetales  and  perennial  dicotyledons, 
showing  inclined  terminal  walls  which  in  one  instance  have  scalari- 
f orm  perforation  of  the  lower  and  in  the  other  the  porous  perforation 
of  the  higher  perennial  types.  Finally,  in/  is  depicted  the  vessel  of 
the  monocotyledons  and  extreme  herbaceous  dicotyledons.  Here 
the  terminal  walls  are  practically  horizontal,  and  the  perforation  is 
porous  (in  the  case  of  the  monocotyledons  invariably  so). 


CHAPTER  DC 
THE  EPIDERMIS 

In  vascular  plants  the  epidermis  is  ordinarily  a  single  layer  of 
cells,  and  this  fact,  together  with  the  external  position  of  the 
epidermis,  makes  the  latter  extremely  easy  to  distinguish.  The 
organization  of  the  epidermis  is  often  strongly  influenced  by  condi- 
tions of  environment,  and  as  a  consequence  the  integumentary 
structures  have  a  value  from  the  evolutionary  standpoint  which  can 
be  very  easily  overestimated.  On  account  of  its  relatively  slight 
phylogenetic  interest  the  epidermal  tissue  will  receive  small  atten- 
tion in  the  present  connection. 

In  the  case  of  the  organs  of  plants  normally  exposed  to  air  and 
not  to  water  or  earth  we  find  the  integumentary  structures  con- 
sisting of  a  well-marked  layer  of  cells,  which  ordinarily  remains 
single  and  is  characterized  by  the  presence  of  stomata  or  pores. 
The  cells  of  the  epidermis  in  lower  forms  often  contain  chloro- 
plastids,  while  in  higher  forms  chlorophyll-containing  plastids  are 
usually  found  only  in  water  plants  or  in  those  inhabiting  shade. 
Plastids  or  masses  of  organized  protoplasm  in  the  epidermal  cells  of 
the  higher  plants  are  not  indeed  rare,  but  usually  when  found  in  the 
superficial  layer  they  lack  color  and  consequently  are  included 
in  the  category  of  leucoplastids.  The  chloroplastids  of  the  epi- 
dermal structures  of  Pteridophyta  may  possibly  indicate  the  primi- 
tive presence  of  these  bodies  in  the  integumentary  tissues  of  the 
higher  plants,  but  they  are  perhaps  equally  susceptible  of  inter- 
pretation as  a  response  to  a  shaded  or  damp  habitat.  The  presence 
or  absence  of  chloroplastids  in  the  epidermal  elements  cannot  con- 
sequently receive  a  very  high  valuation  from  the  standpoint  of 
determination  of  the  relative  phylogenetic  position  of  groups  of 
vascular  plants.  The  situation  is  particularly  obscure  on  account 
of  the  lack  of  bearing  of  fossil  evidence  on  the  question,  both 
because  plastids  are  rarely  preserved  in  the  cells  of  extinct  plants 
and  because,  even  if  they  were,  it  would  be  impossible  to  distin- 

126 


THE  EPIDERMIS 


127 


guish  between  chloroplastids  and  leucoplastids  in  the  altered  state 
which  necessarily  results  from  the  condition  of  fossilization. 

In  some  instances  the  epidermis  commonly  becomes  a  multiple 
layer  often  known  as  water  tissue.  This  is  particularly  the  case 
with  plants  of  different  systematic  affinities  which  are  exposed  to 
extremely  arid  conditions.  Ficus  elastica  affords  an  excellent 
illustration  of  the  multiplication  of  the  epidermal  layer  in  leaves 


FIG.  95. — Epidermis  of  the  Urticaceae.     Explanation  in  the  text 

of  xerophytic  habit.  Here  the  epidermis,  which  in  the  young  con- 
dition consists  of  a  single  stratum,  as  the  leaf  matures  becomes 
divided  into  several  stories  of  cells,  all  characterized,  like  the  epider- 
mis from  which  they  are  derived,  by  the  absence  of  chloroplastids. 
Another  feature  which  especially  distinguishes  the  epidermal 
structures  of  the  Urticaceae  and  Acanthaceae  is  the  presence 
of  deposits  of  carbonate  of  lime,  generally  in  the  form  of  acinose 
masses  supported  on  a  peduncle  from  one  side  of  the  cell.  In 
urticaceous  leaves  which  are  mesophytic  in  their  environment, 
such  as  the  nettle  or  hemp,  the  structures  under  discussion,  known 
as  cystoliths,  are  found  in  the  outermost  cells  of  the  plant — that  is, 


128  THE  ANATOMY  OF  WOODY  PLANTS 

in  the  elements  of  the  simple  epidermis.  In  Ficus  elastica,  on  the 
other  hand,  the  multiplication  of  the  epidermal  layer  has  resulted 
in  the  imbedding  of  the  cystoliths  deeply  in  the  substance  of  the 
water  tissue.  Cystoliths  are  a  valuable  diagnostic  feature  in  the 
case  of  the  Urticaceae  and  Acanthaceae. 

In  many  gymnospenns  the  epidermal  layer  is  reinforced, 
frequently  in  the  leaf  and  more  rarely  and  only  in  the  most 
ancient  representatives  of  the  group  in  the  stem,  by  a  layer  of 
colorless  and  generally  strongly  thickened  cells  known  as  the  hy- 
poderma.  It  is  not  at  the  present  time  clear  that  the  hypoderma 
so  called  in  gymnospermous  leaves  is  derived  from  the  epidermis 
by  the  multiplication  of  its  cells,  but  the  structure  in  question  is 
most  conveniently  discussed  under  this  head.  This  layer  occurs 
very  distinctly  in  the  leaves  of  the  cycads,  Ginkgo,  and  the  conifers, 
and  deepens  the  strata  of  colorless  cells  bounding  the  surfaces  of  the 
leaf.  In  older  types  the  hypodermal  layers  tend  to  become  ribbed, 
and  in  the  leaves  of  Mesozoic  pines  and  other  conifers,  as  well  as  of 
the  Cordai tales  of  the  Paleozoic,  this  condition  is  often  very  marked. 
In  the  Cordaitales,  Cycadofilicales,  and  associated  groups  the  ribbed 
condition  of  the  tissues  supporting  the  epidermis  is  often  strongly 
developed  in  the  stem  organs.  It  is  not,  however,  by  any  means 
settled  whether  the  hypodermal  layers  are  of  common  origin  with 
the  epidermis  and  hence  belong  to  the  same  histological  category. 

In  plants  exposed  to  extreme  drought,  physical  or  physiological, 
the  epidermis  responds  not  only  by  the  frequent  multiplication  of 
its  layers  but  also  by  the  development  of  a  well-marked  cuticle  and 
by  the  cuticularization  of  the  outer  region  of  the  external  walls  of  its 
cells.  This  condition  is  well  developed  in  plants  of  desert  habit 
or  in  those  rooted  in  poisonous  soil,  even  when  moist  conditions  are 
present.  In  aquatics,  on  the  other  hand,  the  cuticular  structures 
are  poorly  differentiated  and  may  be  lacking  even  in  the  case  of 
the  spores,  which  more  constantly  than  any  other  structures  of  the 
higher  plants  maintain  a  cuticularized  exine  or  outer  coat.  The 
question  of  the  development  of  cuticle  in  plants  is  of  some  interest 
from  the  standpoint  of  past  climatic  conditions  on  the  surface  of 
our  earth,  although  even  here  it  is  not  conclusive  on  account  of  the 
nearly  identical  influences  of  dry  and  merely  poisonous  substrata. 


THE  EPIDERMIS  129 

Structurally  the  value  of  the  cuticle  for  purposes  of  identification  or 
as  a  basis  of  evolutionary  speculations  can  easily  be  exaggerated. 
There  is  little  doubt  that  it  shares  with  the  general  external  form  of 
plants  the  shortcojning  of  unreliability,  which  results,  as  in  the 
case  of  formal  characters,  from  close  connection  with  physiological 
necessities. 

The  epidermis  is  not  only  a  limiting  membrane  of  plants,  but  is 
likewise  charged  with  the  important  office  of  bringing  about 
regulated  interchanges  between  internal  spaces  and  the  outside 
air.  The  mechanism  which  performs  the  function  of  facilitating 
gaseous  and  other  interchanges  is  the  stoma.  This  structure  con- 
sists of  a  pair  of  guard  cells  variously  organized  to  meet  conditions  of 
environment  and  bounding  a  pore,  the  stoma  proper.  The  guard 
cells  in  cases  where  they  are  actually  functional  have  their  inner 
and  outer  walls  so  thickened  that  the  accumulation  of  osmotic 
pressure  within  the  cells  leads  to  their  divergence  in  the  middle 
line  with  the  resultant  opening  of  the  pore.  No  matter  what  may 
be  the  conditions  in  regard  to  the  presence  or  absence  of  chloro- 
plastids  in  the  general  epidermal  cells,  the  guard  cells  are  always 
provided  with  green  corpuscles  and  even  under  conditions  of 
starvation  retain  a  modicum  of  starch  after  this  substance  has 
disappeared  in  the  rest  of  the  cells  of  the  leaf.  In  plants  exposed  to 
a  high  degree  of  drought  and  insolation  the  guard  cells  of  the  stoma ta 
become  very  much  thickened  and  respond  to  the  stimulation  of 
light  and  moisture  only  under  extreme  conditions.  A  further 
safeguarding  of  the  epidermal  pores  or  stomata  often  results  from 
their  being  depressed  below  the  surface  of  the  leaf  or  sheltered 
under  a  hairy  protection.  Plainly,  under  the  last-described  con- 
ditions the  loss  of  water  from  the  stomata  will  be  much  less 
than  in  the  unsheltered  condition.  The  guard  mechanism  in 
plants  exposed  to  extreme  drought  is  sometimes  throttled,  as  it 
were,  by  a  surrounding  zone  of  cells  which  differ  from  the 
ordinary  epidermal  elements  in  possessing  chloroplastids.  Cells 
of  this  type  are  known  as  accessory  cells.  The  accessory  device 
is  variously  related  to  the  guard  cells,  the  free  movement  of 
which  it  serves  to  check.  Very  frequently  the  braking  mechan- 
ism consists  of  a  collar  surrounding  the  guard  cells,  while  in  a  large 


130  THE  ANATOMY  OF  WOODY  PLANTS 

number  of  instances  the  accessory  cells  lie  over  the  guard  cells, 
so  that  the  latter  are  nearly  or  quite  shut  off  from  the  surface  of  the 
leaf. 

In  many  cases  the  guard  cells  have  walls  which  differ  chemically 
in  different  regions.  In  the  pine,  for  example,  the  walls  of  the 
elements  inclosing  the  pore  of  the  stoma  along  their  inner  margins, 
where  they  actually  abut  on  the  stomatic  aperture,  and  on  the 
opposite  sides,  where  they  are  in  direct  contact  with  the  adjacent 
cells  of  the  epidermis,  are  in  a  condition  of  pectic  cellulose  and 
absorb  strongly  certain  of  the  hematoxylin  stains.  It  has  been 
quite  generally  observed  in  physiological  investigations  that  the 
inner  and  outer  walls  of  the  stomatic  guard  cells  are  extremely 
viable  to  water — so  much  so,  hi  fact,  that  a  deficient  water  supply 
in  plants  flourishing  under  ordinary  garden  conditions  results  in 
the  rapid  loss  of  turgescence  in  these  elements. 

The  epidermal  structures  are  frequently  characterized  by  out- 
growths known  as  hairs.  These  are  most  generally  composed  of 
comparatively  few  cells,  but  in  some  instances  may  become  very 
complicated  in  their  organization.  When  it  is  clear,  in  spite  of  any 
degree  of  elaboration  and  magnitude,  that  the  organs  are  purely 
epidermal  hi  their  origin,  they  are  properly  designated  as  hairs 
or  trichomes.  If  tissues  of  the  fundamental  or  fibrovascular 
systems,  or  both  together,  enter  into  the  structure  of  the  processes, 
they  are  known  as  emergences,  provided  they  do  not  come  more 
accurately  under  the  caption  of  modified  leaves  or  lobes  of  leaves, 
or,  finally,  of  branches.  The  trichomes  are  not  usually  of  a  high 
value  hi  connection  with  evolutionary  anatomy,  since  they  are 
subject  to  a  very  considerable  degree  of  variability  in  accordance 
with  conditions  of  environment. 

The  sporangia  of  vascular  plants  are  sometimes  considered 
to  belong  in  the  category  of  trichomes,  but  it  is  very  difficult  to 
bring  them  consistently  under  this  heading.  While  it  may  be 
maintained  with  a  certain  degree  of  force  that  the  spore  sacs  of  the 
polypodiaceous  ferns  are  of  the  nature  of  hairs,  since  they  are 
clearly  entirely  derived  from  the  superficial  cells  of  the  leaf,  it  is 
difficult  to  homologize  them  on  this  interpretation  with  the  spore- 
producing  members  of  the  lower  and  more  primitive  groups  of 


THE  EPIDERMIS  131 

Pteridophyta.  It  must  be  confessed  that  the  actual  organization 
of  the  sporangium  is  a  better  indication  of  its  morphological  sig- 
nificance than  is  any  relation  in  origin  to  any  particular  tissue 
system  or  any  combination  of  tissue  systems.  Taking  the  vascular 
plants  as  a  whole,  it  seems  evident  that  the  sporangium  probably 
antedated  the  clear  differentiation  of  the  tissue  systems  and  is 
accordingly  best  regarded  as  an  organ  sui  generis  and  as  belonging 
consequently  to  a  distinct  anatomical  category.  It  will  be  shown 
in  what  follows  that  there  is  good  evidence  in  certain  instances 
that  tissue  systems  other  than  the  epidermis  enter  into  the  organiza- 
tion of  the  sporangial  structures  of  the  seed  plants. 

In  conclusion  it  may  be  stated  that  the  epidermal  structures 
show  an  exceptional  degree  of  plasticity  to  the  molding  influences  of 
environment,  a  condition  which  makes  them  of  less  value  from  the 
phylogenetic  standpoint.  This  highly  variable  system  constitutes 
the  boundary  between  the  organism  and  its  surroundings,  whether 
fluid,  gaseous,  or  solid.  It  consists  normally  of  a  single  layer  of 
cells  which  in  certain  drought-resisting  plants  is  multiplied  to  con- 
stitute what  is  ordinarily  called  water  tissue.  The  epidermis  is 
perforated  by  pores  known  as  stomata  and  produces  outgrowths 
known  as  trichomes  or  hairs.  The  former  structures  consist  of  a 
pair  of  cells  usually  highly  responsive  to  the  combined  presence  of 
light  and  carbon  dioxide  and  surrounding  an  occlusible  pore.  True 
stomata  are  confined  to  the  spore-producing  generation  of  the 
higher  plants  (from  the  Bryophyta  upward).  In  spite  of  erroneous 
statements  to  the  contrary,  in  the  case  of  certain  badly  preserved 
fossil  plants  there  are  never  more  than  two  guard  cells  related 
to  a  stomatic  opening.  Assertions  contradicting  this  are  based 
on  the  interpretation  of  accessory  cells  as  guard  cells,  where  the 
former  overlie  the  elements  guarding  the  stomatic  pore.  Neither 
the  shape  of  the  epidermal  cells  nor  the  organization  of  the  stomata 
nor  the  structure  of  the  hairs  or  trichomes  can  be  regarded  as  having 
a  very  high  importance  from  the  standpoint  of  the  anatomical 
identification  of  extinct  plants  in  the  absence  of  the  confirmatory 
evidence  presented  by  external  form  and  internal  anatomy. 


CHAPTER  X 
THE  FUNDAMENTAL  TISSUES 

It  has  been  indicated  above  that  the  epidermal  tissues  constitut- 
ing the  external  boundary  of  the  plant  toward  the  outer  medium 
have  relatively  slight  value  from  the  standpoint  of  evolutionary 
anatomy.  The  same  statement  may  be  made  (with  less  emphasis) 
concerning  the  category  of  cellular  organization,  known  as  the 
fundamental  tissues.  This  group  is  on  the  whole  much  more 
extensively  developed  and  more  clearly  defined  in  the  lower  forms 
of  vascular  plants  than  in  the  case  of  those  higher  in  the  scale. 
The  more  massive  presence  and  the  more  distinct  individualization 
in  the  fernlike  representatives  of  vascular  plants  naturally  carry 
with  them  a  higher  degree  of  histological  differentiation.  The 
fundamental  system  is  definitely  bounded  internally  by  the  endo- 
dermis  (phloeoterma  of  Strasburger)  in  most  of  the  lower  groups  of 
plants,  while  in  more  advanced  vegetable  organisms  the  limits  be- 
tween the  fundamental  and  fibrovascular  systems  are  often  obscurely 
indicated  except  in  that  most  conservative  of  all  organs,  the  root. 
Externally  the  limits  of  the  fundamental  system  are  usually  clear 
unless  the  multiplication  of  the  epidermal  layer  conceals  the  situa- 
tion. Occasionally  in  roots  the  fundamental  system  is  sharply 
marked  off  toward  the  epidermal  system  by  the  presence  of  a  well- 
developed  limiting  layer,  the  exodermis,  which  externally  is  the 
counterpart  of  the  endodermis,  indicating  the  internal  confines  of 
the  cortex. 

The  accompanying  figure  (Fig.  96)  of  the  subterranean  stem 
of  the  bracken  fern  will  illustrate  both  the  definite  limits  and  the 
considerable  degree  of  specialization  and  consequent  physiological 
importance  of  the  fundamental  tissues.  It  is  clear  that  within  the 
epidermis  and  external  to  the  fibrovascular  bundles  there  is  present 
a  large  amount  of  cellular  substance.  This  is  distinctly  differ- 
entiated into  two  categories — namely,  sclerotic  or  supporting 
tissues  and  storage  parenchyma.  The  former  is  present  in  two 

132 


THE  FUNDAMENTAL  TISSUES 


133 


FIG.  96. — Rootstock  of  the  bracken  fern 


distinct,  more  or  less  continuous,  zones:  an  outer  one  lying  imme- 
diately under  the  epidermis  and  somewhat  interrupted  opposite 
the  angular  projections  on  the  sides  of  the  rhizome,  and  an  inner 
one  coming  between 
the  large  medullary 
and  the  smaller  ex- 
ternal series  of 
bundles.  The  stor- 
age cells,  which  are 
characterized  in  the 
figure  by  their  walls 
and  in  nature  by 
the  large  amount  of 
starch  present  in 
their  cavities,  oc- 
cupy all  the  trans- 
verse section  not 
taken  up  by  the  epi- 
dermis,  the  fibro- 

vascular  system,  and 
the  sclerotic  bands 
just  described.  The 
mechanical  require- 
ments of  the  stems 
of  many  lower  vas- 
cular plants  are  pro- 
vided for  by  skeletal 
structures  present  in 
the  fundamental 
system.  This  situa- 
tion is  common  to 
both  the  Pteropsida 
and  the  Lycopsida. 

.  97. — btem  of  Lepidodendron  Spencen  ,  ,         ,      .. 

The  truth  of  this 
statement  is  revealed  by  the  figure  presented  of  the  stem  of  a 
lepidodendrid,  a  very  ancient  representative  of  the  group  of  club 
mosses  or  Lycopsida.  Here  zones  of  thick-walled  skeletal  tissues 


134 


THE  ANATOMY  OF  WOODY  PLANTS 


belonging  to  the  fundamental  system  can  readily  be  distinguished. 
It  may  be  stated  in  a  general  way  that  the  higher  vascular 
plants  have  their  skeletal  tissues  developed  from  the  progressive 

mechanical  differentia- 
tion of  the  primitively 
solely  water-conducting 
fibrovascular  system,  in 
contrast  to  the  Pteri- 
dophyta,  in  which  the 
mechanical  function 
mainly  resides  in  the 
fundamental  system. 
In  the  higher  vascular 
groups  not  only  does 
the  mechanical  principle 
find  its  best  expression 
in  connection  with  the 
fibrovascular  system, 


FIG.  98. — Transverse  section  of  young  root 
of  the  balsam  fir.     Explanation  in  the  text. 


but  the  fundamen- 
tal category  of 
tissues  becomes 
relatively  insignifi- 
cant in  amount, 
particularly  in 
stems  with  peren- 
nial growth.  Fur- 
ther, the  bound- 
aries between  the 
fibrovascular  and 
fundamental  tis- 
sues have  gen- 
erally disappeared 
through  the  de- 
generacy of  the 
endodermis. 

In  the  case  of  the  root  of  all  vascular  plants  the  limit  between  the 
conductive  strands  and  the  fundamental  system  is  clearly  marked 
from  the  lowest  forms  to  those  at  the  very  summit  of  the  vascular 


FIG.  99. — Older  root  of  the  balsam  fir 


THE  FUNDAMENTAL  TISSUES  135 

series.  This  situation  is  in  accordance  with  the  marked  conserva- 
tism of  roots.  The  clear  boundaries  between  fibrovascular  and 
fundamental  structures  are,  for  example,  as  well  marked  in  the 
cycads,  conifers,  and  angiosperms  as  they  are  in  the  case  of  the 
lycopods,  ferns,  and  Equisetales.  In  the  conifers  and  many  dicot- 
yledons the  fundamental  group  of  tissues,  although  well  developed 
in  the  young  condition  of  the  organ,  later  entirely  disappears. 
The  accompanying  illustrations  (Figs.  98  and  99)  of  the  root  in  the 
balsam  fir  make  the  truth  of  this  statement  sufficiently  clear.  In 
the  first  of  the  two  figures  the  organ  is  shown  in  a  young  condition, 
as  is  evidenced  by  the  slight  degree  of  development  of  the  woody 
cylinder.  A  clear  boundary  separates  the  outer  region  of  the  root 
from  the  tissues  belonging  to  the  fibrovascular  system.  This  is  the 
endodermis  or  innermost  layer  of  the  fundamental  system.  The 
organization  of  the  second  figure  shows  a  further  degree  of  develop- 
ment of  the  fibrovascular  system,  while  the  fundamental  tissues, 
inwardly  limited  by  the  endodermis,  have  begun  to  shrivel  and  break 
away  from  the  surface  of  the  fibrovascular  cylinder.  The  destruc- 
tion of  the  jacket  of  fundamental  tissues  is  the  result  of  the  appear- 
ance of  a  corky  layer  immediately  within  the  endodermis;  and  this, 
impervious  in  its  nature,  cuts  off  the  supply  of  nutrition  to  the  exter- 
nal zone.  This  region  of  the  root  consequently  dies  and  flakes  off. 
In  the  case  of  the  leaf  the  fundamental  tissues  are  of  consider- 
able importance,  since  they  constitute  the  green  substance  or 
mesophyll  of  foliar  organs.  The  limits  between  the  stelar  or 
fibrovascular  strands  in  the  leaf  and  the  surrounding  mesophyll 
are  in  general  much  less  well  marked  than  in  the  root,  but  on  the 
whole  much  more  clearly  indicated  than  in  the  stem.  In  the 
Pteridophyta  in  general  the  endodermis  is  distinctly  devel- 
oped in  the  leaf.  In  the  gymnosperms  the  limits  between  the 
fundamental  system  and  the  fibrovascular  strands  are  already  less 
obvious,  and  in  the  case  of  the  angiosperms  a  somewhat  similar 
condition  is  to  be  observed.  The  usual  situation  justifies  the 
summary  statement  that  in  foliar  organs  the  fundamental  category 
of  tissues  is  always  well  developed  and  is  physiologically  of  great 
importance,  since  it  subserves  the  cardinal  functions  of  transpiration 
and  photosynthesis.  Further,  the  morphological  limitation  of  the 
fundamental  system  is  only  less  distinct  in  leaves  than  it  is  in  roots. 


CHAPTER  XI 
DEFINITIONS  OF  THE  ORGANS 

Not  uncommonly  an  organ  is  defined  as  the  tool  of  a  function. 
Of  course  from  the  evolutionary  point  of  view  this  conception  cannot 
hold,  because  the  same  organ  in  a  plant  often  at  different  times 
subserves  very  diverse  functions.  For  example,  the  stem  may 
function  as  a  leaf  and  the  leaf  as  a  root.  From  the  standpoint  of 
the  doctrine  of  descent  the  value  of  a  particular  organ  in  the  course 
of  evolution  is  assigned,  not  so  much  on  the  basis  of  what  it  does, 
as  on  that  of  what  it  is.  In  other  words,  an  organ  is  known  mainly 
by  its  organization. 

In  the  case  of  the  vascular  plants  there  are  a  number  of  distinct 
structural  units  to  which  the  name  of  organ  is  applied.  It  is  not 
by  any  means  clear  that  the  different  organs  of  plants  were  always 
as  distinct  from  one  another  as  they  are  at  the  present  time.  For 
example,  there  is  some  reason  to  believe  that  the  root  may  manifest 
the  primitive  structural  features  of  vascular  plants,  and  in  the 
course  of  time  the  stem  may  have  become  differentiated  from  the 
root,  arriving  finally  at  that  degree  of  distinctness  which  at  present 
definitely  separates  it  from  the  root.  It  has,  moreover,  often  been 
suggested  in  recent  years  that  the  leaf  of  the  higher  vascular  plants 
was  originally  of  the  nature  of  a  branch  and  that  its  distinguishing 
features  are  the  product  of  later  evolution.  It  would  be  going 
beyond  the  range  of  an  elementary  work  like  the  present  one  to 
discuss  the  question  of  the  origin  of  organs,  particularly  as  the  data 
are  extremely  meager  and  not  always  easy  to  interpret.  Disre- 
garding, consequently,  any  speculations  as  to  the  appearance  of  the 
organs  of  the  higher  or  vascular  plants,  we  may  proceed  at  once 
to  the  enumeration  of  those  which  are  generally  accepted  by 
anatomists. 

The  parts  or  organs  of  the  higher  plants  are  usually  distinguished 
as  three — namely,  root,  stem,  and  leaf.  To  these  may  be  added  a 
fourth,  the  sporangium  or  spore  sac.  Each  of  the  organs  named 

136 


DEFINITIONS  OF  THE  ORGANS  137 

has  its  particular  features  of  organization  and  can  be  traced  as  a 
definite  and  distinct  structure  far  into  the  geological  past  of  our 
existing  plants.  Of  the  organs  the  root  is  the  one  usually  distin- 
guished by  subserving  the  function  of  attachment  to  the  substratum 
and  also  that  of  absorption  of  nutritive  substances  in  solution  from 
the  soil.  In  the  case  of  the  root,  also,  the  direction  of  growth  is 
normally  and  primitively  downward.  The  stem  has  an  upwardly 
growing  axis  which  serves  as  a  support  for  the  other  parts  of  the 
plant.  The  leaf  is  distinguished  in  turn  by  its  usually  flattened 
form,  which  particularly  qualifies  it  for  its  important  function  of 
bringing  the  organism  into  relation  with  light  and  the  gases  of  the 
atmosphere.  The  sporangium  has  the  particular  office  of  producing 
spores  and  may  consequently  with  a  certain  degree  of  appropriate- 
ness be  designated  as  the  organ  of  reproduction.  The  indications 
supplied  above  as  to  the  roles  ordinarily  played  by  the  respective 
parts  of  the  higher  plants  by  no  means  afford  reliable  definitions  of 
those  organs.  Very  frequently  stems,  more  rarely  leaves,  and 
sometimes  even  sporangia  serve  in  connection  with  the  function  of 
attachment,  so  that  the  relation  to  the  substratum  cannot  be 
assigned  as  a  fundamental  and  exclusive  feature  of  the  root. 
Similar  objections  may  be  raised  in  regard  to  the  functional  defini- 
tion of  all  the  parts  or  organs  of  .plants.  In  the  present  connection 
it  will  be  well  to  consider  these  important  categories  of  structure 
from  the  standpoint  of  organization  rather  than  function,  as  that 
procedure  is  most  advantageous  from  the  evolutionary  point  of 
view. 

THE   ROOT 

There  is  good  reason  to  believe  that  the  root  is  on  the  whole 
the  most  primitive  of  plant  organs,  and  it  will  be  shown  in  subse- 
quent chapters  that,  even  if  there  be  room  for  doubt  as  to  its  primi- 
tiveness,  there  can  be  none  as  to  its  conservative  character.  There 
isr  in  fact,  no  organ  of  plants  so  antique  in  its  organization  or  so 
retentive  of  ancestral  traits  as  is  the  root.  Functionally,  as  has 
been  indicated  above,  it  serves  usually  to  connect  the  plant  with 
the  substratum.  Its  unique  and  distinguishing  features,  however, 
are  supplied  by  its  internal  organization.  Roots  are  characterized 
by  two  main  structural  features — namely,  the  possession  of  a  root 


138  THE  ANATOMY  OF  WOODY  PLANTS 

cap  and  a  type  of  fibrovascular  organization  known  as  radial. 
The  root  cap  or  pileorhiza  clearly  differentiates  roots  from  other 
structures  in  the  plant  which  may  happen  under  the  stress  of 
environment  to  assume  a  subterranean  mode  of  existence.  More- 
over, the  root  cap  persists  even  in  those  cases  in  which  the  root 
becomes  aerial  or  aquatic  in  its  habit.  We  need  not  devote  atten- 
tion to  the  mode  of  origin  of  the  pileorhiza,  or  protective  tip  of 
the  root,  as  studies  of  this  nature  are  at  the  present  tune  of  doubt- 
ful morphological  value,  and  the  consideration  of  them  in  an  ele- 
mentary work  is  quite  out  of  the  question.  All  that  need  be  stated 
is  that  the  cap  occupies  the  tip  of  the  root  and  is  continually  renewed 
from  behind,  sometimes,  but  not  invariably,  by  a  well-defined  active 
tissue  known  as  the  calyptrogen.  The  protective  cap  is  character- 
istic of  all  roots  from  the  lowest  to  the  highest  plants.  There  is 
only  one  general  condition  under  which  this  structure  is  ordinarily 
lacking.  In  the  fungus-infected  roots  or  mycorrhizae  of  humus 
plants,  in  which  the  fungal  infection  is  superficial  and  does  not 
notably  invade  the  internal  tissues  of  the  root,  the  root  cap  is 
degenerate  or  absent.  In  the  case  of  the  haustoria  of  certain  root 
parasites,  such  as  the  dodder,  not  only  is  the  root  cap  absent,  but 
also  the  general  condition  of  degeneracy  is  so  marked  that  frequently 
the  only  criterion  of  the  morphological  value  of  the  structure  is  its 
internal  or  endogenous  origin  in  the  axis  of  the  parasite. 

A  salient  feature  of  the  organization  of  the  root  is  furnished 
by  the  fibrovascular  tissues.  Here  the  type  of  fibrovascular  sys- 
tem is  that  known  as  radial.  In  this  condition  the  masses  of 
phloem,  instead  of  surrounding  the  wood  or  xylem  (a  state 
commonly  found  in  the  ferns  and  their  allies)  or  lying  just  outside 
of  it  in  the  same  radius  (seed  plants)  as  in  the  stem  and  leaf,  lie 
in  different  and  distinct  radii.  This  mode  of  relative  disposition 
of  phloem  and  xylem  is  responsible  for  the  term  radial  as  applied  to 
roots,  and  it  is  a  very  important  characteristic  of  their  organization. 
Where  the  phenomenon  of  secondary  growth  in  thickness  is  absent, 
there  is  no  subsequent  modification  of  the  general  relations  of 
tissues  in  roots.  In  the  case  of  roots  with  secondary  growth,  how- 
ever, the  situation  changes  with  the  appearance  of  the  secondary 
xylem  and  phloem.  The  secondary  tissues  begin  to  form  in  definite 


DEFINITIONS  OF  THE  ORGANS  139 

relation  to  the  clusters  of  primary  phloem  in  all  probability  because 
the  foodstuffs  are  provided  by  that  tissue.  The  active  layer 
which  makes  its  appearance  here  continually  adds  new  elements 
on  the  outside  to  the  phloem,  and  on  the  inside  gives  rise  to  the 
secondary  xylem.  As  a  consequence  of  this  situation  the  secondary 
wood,  being  formed  opposite  the  clusters  of  primary  phloem, 
naturally  alternates  with  the  primary  wood.  With  the  beginning 
of  the  secondary  growth  the  root  therefore  abandons  the  radial 
type  of  organization  of  the  primary  structures.  The  lateral  roots 
originating  from  a  given  root  have  their  position  determined, 
however,  by  the  topography  of  the  primary  structures  in  the  main 
root  and  grow  out  in  vertical  rectilinear  rows  corresponding  to  the 
primary  xylem.  But  in  the  monocotyledons  an  exception  to  this 
condition  is  found,  since  each  lateral  root  originates  in  the  interval 
between  two  angles  of  the  primary  xylem. 

Although  the  secondary  structures  of  the  root,  as  has  been 
indicated  above,  are  collaterally  organized,  the  primary  plan  of  the 
root  is  radial.  The  root,  in  fact,  is  the  only  organ  of  the  plant 
for  which  a  single  ground  plan  will  illustrate  the  conditions  found 
in  all  groups  of  plants,  living  or  extinct.  The  extreme  conservatism 
of  the  anatomical  organization  of  the  root  is  an  outstanding  feature 
and,  as  will  be  indicated  in  succeeding  chapters,  renders  it  of  the 
greatest  value  in  working  out  the  evolutionary  sequence  of  the 
various  groups.  Extreme  conservatism,  radial  organization  of 
the  primary  fibrovascular  structures,  the  possession  of  a  root  cap 
or  pileorhiza,  and  an  internal  origin  from  the  surface  of  the  fibro- 
vascular cylinder  are  the  salient  and  important  criteria  of  the  root. 

THE   STEM 

In  the  case  of  the  axis  or  shoot  of  plants  no  such  general  formula 
can  be  arrived  at  as  in  the  root.  Just  as  the  root  is  the  least  change- 
able of  all  the  organs  of  the  plant,  so  the  stem  is  of  all  the  most 
variable.  In  internal  organization  it  varies  greatly  from  the  lower 
and  more  ancient  groups  to  the  higher  and  more  modern.  So 
numerous  are  the  types  of  stem  as  regards  anatomical  structure 
that  they  can  most  profitably  be  discussed  under  the  particular 
groups  of  the  vascular  plants  in  later  chapters  of  the  volume.  In 


140  THE  ANATOMY  OF  WOODY  PLANTS 

the  absence  of  any  general  and  common  anatomical  characteristics 
of  the  stem  for  vascular  plants  as  a  whole,  it  is  necessary  to  define 
it  by  the  important  criteria  supplied  by  the  mode  of  attachment  of 
the  appendages.  The  axis  is  characterized  by  the  fact  that  it  is 
divisible  into  nodes  from  which  the  appendages  normally  take  their 
origin  and  into  segments  more  or  less  elongated  separating  these 
from  one  another  and  known  as  internodes.  By  the  possession  of 
nodes  and  internodes  the  stem  is  at  once  distinguished  from  the 
root  and  the  leaf.  There  are  usually  at  least  two  kinds  of  append- 
ages attached  to  the  stem  in  the  region  of  the  nodes — namely,  the 
leaves  and  secondary  axes  or  branches.  If  the  axis  be  subterranean, 
roots  may  also  make  their  appearance  in  the  region  of  the  nodes 
and  are  dearly  separated  from  the  other  appendages  by  their 
internal  origin.  Both  leaf  and  branch  are  formed  superficially  or 
exogenously  on  the  axis.  The  leaf  and  secondary  axes  or  branches 
are  frequently  related  to  one  another  in  a  quite  definite  manner,  the 
young  branch  appearing  in  the  upper  angle  or  axil  of  the  leaf. 
Further,  the  arrangement  of  the  leaves  and  consequently  of  their 
axial  related  structures,  the  branches,  follows  a  somewhat  definite 
plan.  Where  more  than  one  leaf  occurs  at  a  node,  the  foliar 
structures  are  said  to  be  opposite  or  whorled;  and  where  they  are 
attached  singly  to  the  nodes,  the  arrangement  of  the  leaves  or 
phyllotaxy  is  said  to  be  spiral.  The  spirals  are  of  different  types, 
according  as  the  number  of  foliar  organs  intervening  between  two 
vertically  opposite  leaves  is  greater  or  smaller;  and  the  spiral 
turns  about  the  stem,  drawn  through  the  leaf  bases,  are  more  or  less 
numerous.  In  the  grasses,  for  example,  the  spiral  phyllotaxy 
is  expressed  by  a  fraction  of  which  one  is  the  numerator  and  two  the 
denominator.  In  the  alder  the  fraction  is  one  over  three,  and  in  the 
oak  or  willow  two  over  five.  The  numerator  indicates  the  number 
of  turns  about  the  stem  in  passing  from  one  leaf  to  the  next  one 
vertically  above  it.  The  denominator  is  supplied  by  the  number  of 
leaves  attached  to  the  stem  in  the  interval  between  two  which 
are  vertically  opposite. 

The  stem  is  not  only  extremely  variable  as  regards  its  anatomical 
structure  in  the  various  groups  of  higher  plants,  but  its  external 
form  is  greatly  modified  in  correlation  to  different  conditions  of 


DEFINITIONS  OF  THE  ORGANS  141 

existence  or  various  needs  on  the  part  of  the  plant.  In  general, 
however,  the  possession  of  nodes  and  internodes  serves  as  a  suffi- 
cient diagnostic  character,  since  this  feature  is  rarely  obliterated 
even  by  the  most  extreme  conditions  to  which  the  organ  is  exposed. 

THE   LEAF 

This  organ  has  a  double  importance,  because  it  not  only  sub- 
serves the  vegetative  functions  present  in  the  case  of  root  and  stem, 
but  is  also  charged  with  the  extremely  important  office  of  repro- 
duction. The  leaf  from  the  purely  vegetative  aspect  is  of  con- 
siderable evolutionary  significance,  since,  although  much  less 
conservative  than  the  root,  it  is  much  more  retentive  of  ancestral 
characteristics  than  is  the  stem.  Its  chief  evolutionary  value,  as 
will  appear  in  subsequent  chapters,  is  in  connection  with  the 
unraveling  of  the  relationships  of  the  older  and  lower  gymno- 
sperms.  In  more  modern  types  the  anatomy  of  the  root 
generally  throws  more  light  on  the  relationships  than  that 
furnished  by  the  leaf.  Reproductively  the  foliar  organs  are  of 
great  importance  in  connection  with  taxonomic  arrangement.  In 
fact,  at  the  present  time  the  angiosperms  are  almost  universally 
classified  on  the  features  presented  by  their  reproductive  leaves, 
and  in  the  case  of  the  higher  gymnosperms  anatomical  features  of 
the  vegetative  structures  are  only  beginning  to  receive  adequate 
consideration  in  connection  with  investigations  as  to  evolutionary 
sequence. 

As  a  reproductive  structure  the  leaf  is  best  considered  under  the 
particular  groups  to  be  discussed  in  subsequent  chapters,  as  no 
general  statements  can  be  advantageously  made  at  the  present 
stage.  As  a  purely  vegetative  organ  the  leaf  is  readily  distinguished 
from  the  stem,  of  which  it  forms  an  appendage,  by  the  absence  of 
nodes  and  internodes.  It  is  not  easy  to  summarize  the  anatomical 
structure  of  the  leaf,  since  it  is  to  a  large  degree  variable  in  passing 
from  lower  to  higher  groups.  A  useful  criterion  is  its  dorsiventral 
organization,  which  is  usually  present,  at  least  in  the  region  of  the 
expanded  part  of  the  leaf  or  blade.  Another  outstanding  feature  of 
the  leaf  is  the  fact  that  it  is  an  axillating  organ,  and  as  a  conse- 
quence other  appendages  are  commonly  axillary  to  it.  The 


142  THE  ANATOMY  OF  WOODY  PLANTS 

general  external  form  of  the  foliar  structure  may  reveal  the  presence 
of  a  narrow  base  or  stalk  known  as  the  petiole,  and  this  is  termi- 
nated by  the  broad,  generally  horizontally  placed,  region  called  the 
blade  or  lamina.  The  petiole  often  has  related  to  it  paired  and 
lateral  appendages  known  as  stipules.  Occasionally  an  unpaired 
appendage  of  the  nature  of  a  stipule,  often  called  the  ligule, 
is  present. 

THE    SPORANGIUM 

This  is  the  organ  of  reproduction.  Although  ordinary  vegeta- 
tive parts  under  certain  conditions,  and  particularly  in  herbaceous 
forms,  may  serve  to  perpetuate  the  plant,  still  the  sporangium  is  the 
special  organ  of  reproduction  and  has  the  most  important  evolu- 
tionary significance  in  this  respect.  The  organ  under  discussion 
is  very  accurately  defined  by  the  fact  that  it  produces  spores  and 
is,  indeed,  the  only  structure  in  the  higher  plants  set  apart  for  this 
purpose.  In  some  cases  the  essential  fact  of  sporogeny  is  more 
or  less  concealed  as  the  result  of  the  deep-seated  modifications  con- 
nected with  heterospory  and  the  seed  habit;  but  in  such  instances 
a  study  of  internal  organization  suffices  to  make  the  situation  clear. 
Normally  the  sporangium  is  a  structure  which  is  an  appendage 
of  the  leaf.  Sometimes  it  appears  to  be  axillary,  although  there 
is  no  reason  to  believe  that  this  was  its  primitive  relation  to 
the  foliar  organs.  Very  frequently  the  reproductive  leaf  undergoes 
profound  modification  in  connection  with  the  functions  it  has  to 
perform,  and  these  features  furnish  a  generally  approved  basis  of 
systematic  grouping  for  the  vascular  plants.  They  need  not  be 
considered  in  any  detail  in  an  elementary  work  on  anatomy  and  are 
best  discussed  in  connection  with  particular  groups  in  appropriate 
later  chapters.  Unlike  the  other  organs,  the  sporangium  shows 
no  indications  of  derivation  from  another  structure.  It  has,  in  fact, 
been  suggested  that  the  sporogonium  of  the  lower  bryophytes  is 
the  ancestral  primordial  from  which  all  the  other  organs  have  been 
derived.  Professor  Bower  has  developed  this  idea  in  liis  Origin  of  a 
Land  Flora. 


CHAPTER  XII 


THE  ROOT 

As  has  been  indicated  in  the  preceding  chapter,  the  root  is 
characterized  by  the  possession  of  a  protective  terminal  structure 
known  as  the  root  cap,  by  its  radial  organization,  and  by  its  endoge- 
nous or  internal  mode  of  origin.  The  first  and  the  last  charac- 
teristics are  exclusive  features  of  the  root,  but  that  of  radial 
organization  is  often 
present  in  stems  of 
the  more  primitive 
types.  A  comparison 
of  the  upright  stem  of 
the  common  club 
moss,  Lycopodium 
clavatum,  with  the 
root  of  the  same 
species  makes  the 
truth  of  this  state- 
ment particularly 
apparent.  It  is  clear 
from  Figs.  100  and 
101  that  the  radial 
organization  is  com- 


FIG.    zoo.— Transverse   section   of    the    root  of 

Lycopodium  clavatum. 


mon  to  the  two 
organs  and  that  they 
differ  only  in  the  fact 

that  the  stem  bears  leaves.  This  resemblance  in  anatomical 
structure  between  stem  and  root  is  often  present  in  the  lower 
Lycopodiales  and  furnishes  a  strong  argument,  taken  together 
with  their  <  ncient  occurrence  and  early  decline  as  a  prominent 
element  of  the  earth's  vegetation,  for  the  view  that  the  lycopods 
are  the  most  primitive  plants. 

In  the  great  majority  of  vascular  plants  the  contrast  between 
the  organization  of  the  stem  and  that  of  the  root  is  very  striking. 

143 


144 


THE  ANATOMY  OF  WOODY  PLANTS 


In  the  case  of  a  conifer,  for  example,  the  root  is  clearly  distinct  from 
the  stem  by  the  possession  of  a  woody  cylinder  devoid  of  pith.  The 
primary  wood,  moreover,  in  the  root  is  very  well  developed  and 
forms  a  lens-shaped  mass  in  the  center  of  the  cylinder  which  is 
clearly  centripetal  in  its  development,  since  the  smaller  elements 
are  situated  in  two  or  more  groups  hi  an  external  position.  The 

primitive  organization  of 
the  root  in  contrast  to  the 
more  progressive  organi- 
zation of  the  stem  is  par- 
ticularly well  indicated  in 
connection  with  the  rays. 
In  the  cauline  woody  cyl- 
inder these  appear  to  take 
their  origin  from  the  pith; 
hence  arises  the  com- 
monly employed  appella- 
tion, medullary  rays.  The 
intimate  relation  between 
rays  and  pith  which  appa- 
rently obtains  in  this  case 
is  in  reality  merely  a  sem- 
blance, owing  to  the  fact 
that  the  primary  wood 
has  become  nearly  obso- 
lete hi  stem  organs  in  the  higher  gymnosperms  and  the  angio- 
sperms.  In  the  root  the  primitive  situation  in  which  the  radial 
parenchyma  has  no  relation  whatever  to  medullary  tissues  is  very 
clear.  It  is  obvious  that  the  common  term  medullary  rays  is 
extremely  inappropriate  as  applied  to  the  radial  parenchyma  of 
the  secondary  wood.  The  true  situation  is  not  only  revealed  by 
the  comparison  of  stem  and  root,  but  it  also  becomes  even  more 
apparent  when  the  stem  in  older  types  is  compared  with  that 
found  in  existing  forms.  The  more  detailed  consideration  of  the 
contrasts  in  organization  between  stem  and  root  is  best  deferred 
to  later  pages,  after  the  anatomy  of  the  various  groups  has  been 
discussed. 


FIG.  101. — Transverse  section  of  the  upright 
stem  of  Lycopodiiim  clavatum. 


THE  ROOT 


145 


We  may  now  turn  our  attention  to  the  particular  features  of 
organization  presented  by  the  younger  and  older  root  in  a  conifer. 
In  the  early  stage  of  development  presented  in  Fig.  102  the  fibro- 
vascular  region  is  sharply  separated  from  the  rest  of  the  transverse 


FIG.  102. — Transverse  section  of  a  young  root  of  the  American  larch 

section  by  a  well-marked  endodermis.  Within  this  layer  lies  a 
broad  zone,  the  pericycle,  which  abuts  upon  the  primary  phloem. 
Inside  of  the  primary  phloem  lie  two  bands  represented  as  double 
rows  of  cells  with  protoplasmic  contents.  This  is  the  cambium, 
which  provides  the  elements  of  the  secondary  xylem  and  phloem, 
neither  of  which  can  be  said  as  yet  to  have  come  into  existence.  In 


146  THE  ANATOMY  OF  WOODY  PLANTS 

the  oval  area  between  the  cambial  bands  lies  the  primary  xylem. 
This  consists  of  smaller  elements  at  the  two  external  angles,  the 
protoxylem  strands.  The  development  of  the  primary  wood  is  not 
complete;  hence  there  is  a  considerable  interval  in  the  center 
which  will  be  later  transformed  into  tracheids  of  the  primary 
metaxylem.  In  the  first-formed  elements  of  the  primary  wood  the 
bordered  pits  of  the  tracheids  can  be  plainly  seen.  Subtending  each 
cluster  of  protoxylem  is  a  resin  canal,  a  common  feature  of  the 
organization  of  the  root  in  Picea  and  allied  genera  of  the  Abietineae. 
It  is  manifest  from  the  general  situation  represented  in  the  median 
region  or  central  cylinder  of  the  root  that  the  primary  structures 
are  the  only  ones  conspicuously  in  evidence,  and  of  these  the  wood 
is  still  incomplete.  Further,  there  is  a  clear  indication  of  the 
presence  of  cambial  layers,  which  will  a  little  later  give  rise  to  sec- 
ondary elements  of  both  wood  and  bast.  Surrounding  the  central 
cylinder  or  stele  of  the  root  is  the  cortex,  which  is  limited  internal!}' 
by  the  endodermis  and  externally  abuts  on  the  root-hair-producing 
stratum  of  the  root:  this  is  ordinarily  called  the  piliferous  layer. 
As  the  section  is  taken  very  near  the  apex  of  the  root,  a  few  rather 
poorly  developed  root  hairs  are  seen.  It  has  often  been  wrongly 
asserted  that  there  are  no  root  hairs  present  in  coniferous  roots. 
While  this  condition  may  be  found  in  the  case  of  those  fungus- 
inhabited  radical  organs  known  as  mycorrhizae,  carefully  excavated 
normal  coniferous  roots  nearly  always  show  a  band  of  root  hairs 
in  proximity  to  the  root  cap  or  pileorhiza.  It  will  be  made  clear  in 
what  follows  that  both  root  hairs  and  cortex  are  of  short  duration 
in  coniferous  roots. 

In  the  older  root  marked  differences  present  themselves  both 
in  the  region  of  the  central  cylinder  and  in  the  cortex.  Taking  first 
the  fibrovascular  tissues,  we  find  that  the  pericycle  is  as  well  marked 
as  in  the  younger  condition  of  the  organ,  but  is  now  characterized 
by  the  formation  in  its  outer  region  of  a  zone  of  regularly  arranged 
cells  known  as  the  periderm.  This  layer  has  an  important  influ- 
ence, as  will  be  indicated  later,  on  the  tissues  constituting  the  outer 
region  of  the  root.  It  is  clear  that  the  primary  phloem  has  more 
centrally  been  superseded  by  a  mass  of  tissue  of  very  regular  radial 
arrangement,  the  secondary  phloem,  derived  as  a  result  of  the 


THE  ROOT  147 

external  activity  of  the  cambium.  The  primary  elements  of  the 
phloem  constitute  a  distinct  band  of  collapsed  cells  on  the  upper 
and  lower  sides  of  the  cylinder,  and  this  is  usually  very  conspicuous 
in  sections  of  the  mature  root,  provided  the  organ  has  not  become 
too  old.  Within  the  zones  of  radially  disposed  secondary  phloem 
lies  the  secondary  xylem,  which  has  its  cells  similarly  arranged  in 
series.  The  two  masses  of  secondary  wood,  facing  the  correspond- 
ing aggregations  of  secondary  phloem,  are  joined  internally  with 
the  primary  xylem,  and  they  are  readily  distinguishable  from  this 
by  the  presence  of  rays  and  the  regular  disposition  of  the  tracheary 
elements.  The  primary  xylem  is  now  complete  and  no  longer  shows 
the  hiatus  or  interruption  in  the  center  which  is  found  in  the 
younger  root.  The  two  resin  canals  which  lie  opposite  the  groups 
of  small  tracheids  constituting  the  protoxylem  have  not  collapsed. 
The  primary  wood  in  the  root,  in  contrast  to  the  primary  phloem, 
does  not  break  down,  but  persists  in  maturity.  Meanwhile  in  the 
exterior  or  cortical  region  of  the  root  an  important  change  has 
taken  place  in  consequence  of  the  formation  of  a  layer  of  impervious 
secondary  tissue  known  as  periderm.  This  zone,  by  reason  of  its 
imperviousness,  prevents  the  conveyance  of  water  and  nourishment 
to  the  outer  portion  of  the  root,  and  the  latter  consequently  dies 
and  is  sooner  or  later  exfoliated  as  a  shriveled  mass  of  collapsed 
cells,  as  indicated  in  the  diagram  (Fig.  103).  The  result  of  the  loss 
of  the  cortex  and  piliferous  layer  in  the  older  roots  of  the  conifers 
is  to  bring  the  fibrovascular  tissues  in  immediate  relation  to  the 
soil.  In  this  stage,  however,  they  no  longer  serve  the  function 
of  absorption,  but  only  that  of  conduction,  since  -the  taking  up  of 
water  and  nutritive  substances  in  solution  is  an  activity  of  the 
younger  root  when  it  is  still  provided  with  root  hairs. 

The  subject  of  the  structure  and  development  of  the  root  is  so 
important  from  the  evolutionary  standpoint  that  it  is  advisable 
to  supplement  the  description  furnished  in  the  two  preceding  some- 
what diagrammatic  drawings  (Figs.  102  and  103)  of  the  organization 
of  the  root  with  actual  photographic  reproductions  of  the  struc- 
tures present.  In  order  that  the  general  validity  of  the  principles 
involved  may  be  made  apparent,  a  different  genus  of  the  conifers 
has  been  purposely  chosen.  The  diagrams  (Figs.  102  and  103)  refer 


148 


THE  ANATOMY  OF  WOODY  PLANTS 


to  the  genus  Larix,  while  the  actual  photomicrograms  illustrate  the 
fir  (Abies).  In  the  first  of  these  (Fig.  104)  is  depicted  the  root  in  a 
younger  stage  of 'development.  The  general  situation  is  clearly 
identical  with  that  in  the  larch,  since  the  central  region  of  the  root 
is  sharply  set  off  from  the  cortex  by  the  presence  of  a  well-marked 
endodermis,  a  structure,  by  the  way,  not  found  in  any  organ  of  the 
genus  but  the  root.  In  the  central  cylinder  may  be  seen  the  broad 


FIG.  103. — Transverse  section  of  an  older  root  of  the  American  larch, 
tion  of  this  and  preceding  figure  in  the  text. 


Explana- 


encircling  pericycle,  bounding  the  primary  phloem  above  and  below. 
Within  the  first-formed  phloem  lies  the  mass  of  primary  wood,  not 
as  yet  completed  in  the  central  region,  and  thus  clearly  indicating 
the  centripetal  order  of  development  of  the  elements  of  that  portion 
of  the  woody  structures.  In  the  midst  of  the  primary  xylem  is 
situated  a  resin  canal  which  is  not  yet  fully  developed.  The  singu- 
larity of  number  and  the  axial  position  of  the  secretory  canal 
furnish  practically  the  only  features  of  distinction  from  the  cor- 
responding structures  in  the  case  of  the  larch,  where  the  plural 


THE  ROOT 


149 


resin  canals  are  found  exclusively  facing  the  clusters  of  protoxylem 
and  are  never  present  in  an  axial  position. 

In  Fig.  105  an  older 
stage  of  the  root  of 
Abies  is  presented. 
Here  the  development 
of  the  primary  wood  is 
completed,  and  as  a  con- 
sequence the  axial  resin 
canal  stands  out  more 
distinctly.  The  second- 
ary activity  has  pro- 
ceeded so  far  that  there 
is  now  a  considerable 
zone  of  radially  seriate 

xylem  and  phloem.    The  FIG.  104.— Young  root  of  Abies  balsamea 

primary  phloem  has  col- 
lapsed. The  ap- 
pearance of  a  layer 
of  cork  or  periderm 
with  radially  dis- 
posed elements  is 
obvious  in  the  re- 
gion of  the  endo- 
dermis,  and  the 
activity  in  this  tis- 
sue is  the  cause  of 
the  death  and  the 
ultimate  shedding 
of  the  external  or 
cortical  region  of 
the  root.  It  is 
quite  clear,  as  has, 
indeed,  been 
pointed  out  above, 

that  the  radial  parenchyma  of  the  root  of  the  conifers  can  in  no  wise 
be  appropriately  designated  medullary  rays,  since  the  structures 


FIG.  105.— Photograph  of  an  older  root  of  Abies 
balsamea.  Description  of  this  and  foregoing  figure  in 
the  text. 


150  THE  ANATOMY  OF  WOODY  PLANTS 

in  question  have  no  relation  whatsoever  to  the  pith,  which  in  this 
case  is  non-existent.  On  general  evolutionary  grounds  there  are 
the  best  of  reasons  for  regarding  the  type  of  organization  present 
in  the  root  of  the  conifers  and,  in  fact,  of  the  gymnosperms  in 
general  as  more  primitive  than  that  which  characterizes  the  stem, 
and  consequently  of  great  value  from  the  standpoint  of  the  doctrine 
of  descent. 


FIG.  106. — Root  of  Osmunda  cinnamomea.     Explanation  in  the  text 


The  root  of  the  ferns  and  their  allies  differs  from  that  found  in 
the  gymnosperms  only  by  the  absence  of  secondary  tissues.  In 
Fig.  106  is  shown  the  root  in  Osmunda.  Here  central  cylinder  and 
cortex  are  clearly  delimited  by  the  endodermis,  composed  of  cells 
with  dark  contents.  Within  this  boundary  lie  the  two  masses  of 
phloem,  and  in  the  intervening  region  is  seen  the  primary  xylem. 
This  consists  externally  of  narrow  primitive  elements  which  are 
continued  toward  the  center  by  the  progressively  broader  tracheids 
of  the  metaxylem.  No  secondary  activities  subsequently  modify 


THE  ROOT  151 

the  topography  of  the  root  in  any  living  representative  of  the  fern 
alliance  or  of  the  Pteridophyta  in  general. 

The  structures  of  the  root  in  the  dicotyledonous  angiosperms 
are  appropriately  considered  at  this  stage.  Since  the  early  condi- 
tion of  development  of  the  root  has  been  already  discussed  and 
figured  in  detail  for  the  conifers,  it  will  not  be  necessary  to  enlarge 
upon  this  phase  in  the  present  connection,  for  the  underlying 
principles  involved  are  the  same  in  both  conifers  and  dicotyledons. 
It  will  be  convenient  to  begin  with  a  woody  or  arboreal  root. 


FIG.  107. — Transverse  section  of  root  in  Alnus  incana.    Explanation  in  the 
text. 

Fig.  107  illustrates  the  organization  of  an  older  root  in  Alnus  incana. 
Toward  the  right  appears  a  layer  of  periderm  bounding  the  outside 
of  the  organ.  Within  lies  the  cortex,  which  terminates  at  a  layer 
of  thick-walled  cells,  the  pericycle.  Within  the  pericyclic  layer, 
marking  the  outside  of  the  fibrovascular  cylinder,  lies  the  phloem, 
not  depicted  in  detail.  Then  comes  the  cambium,  followed  in- 
ternally by  the  secondary  wood,  in  which  conspicuous  rays  are 
present.  Toward  the  extreme  left  of  the  illustration  lies  the  five- 
angled  star  indicating  the  topography  of  the  primary  xylem.  The 
stellate  aggregation  of  elements  shows  at  each  angle  of  its  five 
points  groups  of  cells  of  smaller  size,  the  clusters  of  protoxylem 
consisting  of  spiral  tracheids.  The  mass  of  metaxylem  into  which 


152 


THE  ANATOMY  OF  WOODY  PLANTS 


the  angles  of  protoxylem  become  confluent  by  continued  centripetal 
development  has  interspersed  with  its  elements  a  varying  amount 
of  parenchyma.  Vessels  are  present  in  the  primary  five-angled 
region  of  the  wood,  but  they  do  not  so  conspicuously  differ  from 
the  tracheids  in  transverse  section  as  is  the  case  with  the  similar 
elements  of  the  secondary  wood. 

Two  features  are  particularly  worthy  of  note  in  the  present 

connection.  First  of 
all,  the  layer  of  peri- 
derm  does  not  occupy 
so  deep  a  position  in 
the  root  as  in  the  larch 
and  fir  shown  above. 
As  a  consequence  the 
central  cylinder  does 
not  generally  become 
stripped  of  its  cortical 
envelope  as  has  been 
found  in  the  conifers. 
Another  interesting 
feature  is  presented  by 
certain  modifications 
in  the  rays  in  relation 
to  the  star-shaped 
mass  of  primary  wood. 

This  situation  is  more  easily  understood  by  reference  to  the 
accompanying  photographs.  The  first  illustration  (Fig.  108) 
represents  a  transverse  section  of  a  mature  root  of  Alnus 
japonica.  The  secondary  wood  is  characterized  by  certain 
modifications  in  the  structure  of  the  radii  corresponding  to 
the  angles  of  the  primary  wood.  These  consist  of  clusters,  more 
or  less  pronounced,  of  rays  of  greater  width  than  the  uniseriate 
condition  found  in  the  wood  at  large,  among  which  the  vessels, 
characteristic  of  the  organization  of  the  remainder  of  the  xylem, 
are  absent.  In  other  words,  aggregate  rays  are  plainly  present  in 
the  secondary  wood  and  are  quite  clearly  related  to  the  groups 
of  protoxylem  marking  the  angles  of  the  primary  xylem.  Since 


FIG.  108. — Transverse  section  of  root  of  Alnus 
japonica. 


THE  ROOT 


153 


the  secondary  roots  definitely  take  their  origin  from  the  same 
clusters  of  protoxylem  which  are  subtended  by  aggregate  rays  in 
the  secondary  wood,  it  follows  that  the  secondary  roots  are  corre- 
lated with  aggregate  rays  precisely  as  the  traces  of  the  leaf  are 
imbedded  in  similar  modifications  of  the  radial  parenchyma  in 
the  case  of  the  stem.  Aggregate  rays  are  much  more  commonly 
found  in  the  root  than  they  are  in  the  shoot  organs,  a  consequence 
of  the  conserv- 
atism of  the  root, 
which  is  to  be 
emphasized  in  a 
later  chapter.  The 
intimacy  of  the  re- 
lation between  the 
aggregate  ray  and 
the  trace  of  the 
secondary  root  can 
be  readily  inferred 
from  the  inspec- 
tion of  Fig.  109,  a 
photograph  of  a 
transverse  section 
of  the  root  oiAlnus 
japonica.  It  is 
clear  that  the  trace 
is  related  to  a  mass 
of  enlarged  rays  among  which  vessels  are  conspicuous  by  their 
absence,  precisely  as  in  the  leaf  trace  illustrated  in  Fig.  130, 
page  177.  It  thus  becomes  clear  that  aggregate  rays  may  be 
a  feature  of  organization  of  the  root  as  they  are  of  the  stem,  and 
that  in  both  organs  the  clusters  or  congeries  of  rays  are  most 
conspicuously  developed  in  relation  to  the  appendages. 

The  secondary  structures  of  the  root  are  characterized  by  the 
same  three  types  of  multiseriate  rays  found  in  the  wood  of 
the  stem  as  figured  and  diagrammed  in  chapter  vi.  In  the  case 
of  the  root,  however,  the  primitive  aggregate  condition  which  pre- 
cedes both  the  compound  and  diffuse  types  tends  strongly  to  persist. 


FIG.  109. — Transverse  section  of  part  of  same,  more 
highly  magnified.  Description  of  this  and  last  figure  in 
the  text. 


154 


THE  ANATOMY  OF  WOODY  PLANTS 


The  truth  of  this  statement  can  be  made  apparent  by  reference  to 
the  oak  and  the  birch  which  represent,  respectively,  in  the  species 

found  in  the 
Northern  United 
States  and  Canada 
the  complete  ex- 
emplification of 
the  compound  and 
diffuse  types  of 
rays.  In  Fig.  noa 
and  Fig.  nob  are 
shown  tangential 
views  of  the  wood 
of  the  stem  and  the 
root  of  Quercus 
rubra,  the  red  oak. 
In  each  appear 
large  rays.  Above 
is  the  illustra- 
tion of  the  con- 
dition in  the 
stem,  and  here 
the  ray  is  obvi- 
ously of  the  type 
described  in  an 
earlier  chapter 
as  compound. 
Below,  the  con- 
dition typical  for 
the  root  is  indi- 
cated, and  it  is 
here  equally  clear 
that  the  large  ray 
is  made  up  of  a  con- 
geries of  smaller 

FIG.  no  a  and  b.— Tangential  sections  of  large  rays  in       rays      Separa  ted 
stem  and  root  of  Quercus  rubra.    Explanation  in  the  text.       from    one    another 


THE  ROOT 


155 


by  fibers.  The  state  of  the  ray  figured  for  the  root  is  the  primi- 
tive one  for  the  stem  and  the  ancestral  one  for  the  genus  Quercus, 
as  evidenced  by  its 
tropical  and  sub- 
tropical represent- 
atives  (live  or 
evergreen  oaks) 
which  typically 
manifest  the  pres- 
ence of  aggregate 
rays.  In  the  genus 
Betula  the  same 
situation  presents 
itself  with  refer- 
ence to  the  aggre- 
gate and  diffuse 
rays.  In  the  stem, 
as  is  shown  in 
Fig .  iiia,  the 
multiseriate  rays 
are  diffused 
throughout  the 
structure  of  the 
wood,  while  in  the 
root  aggregations 
of  such  rays  are  re- 
lated to  the  out- 
going traces  of  the 
secondary  roots, 
Fig.  i nb.  Similar 
conditions  are  of 
widespread  occur- 
rence, and  the  root 
clearly  furnishes 
evidence  as  to  the 


nature  of  primi- 
tive   organization 


FIG.  ma  and  b. — Transverse  section  of  rays  in  stem 
and  root  in  Betula  papyrifera.    Explanation  in  text. 


THE  ANATOMY  OF  WOODY  PLANTS 


in  general  in  vascular  plants.  The  illustration  supplied  by  the 
rays  serves  as  an  exemplification  of  the  original  condition  of  the 
root  as  regards  its  secondary  structure.  Additional  examples  will 
present  themselves  repeatedly  in  the  subsequent  chapters.  In  the 
case  of  the  primary  organization  the  uniformity  of  the  radial  struc- 
ture presented  by  the  root  throughout  the  vascular  series,  in  contrast 

to  the  great  variety  of 
types  exemplified  by  the 
stem  both  in  regard  to 
the  arrangement  and 
mode  of  development  of 
the  fibrovascular  ele- 
ments, vouches  even 
more  emphatically  for 
the  value  of  the  anat- 
omy of  this  organ  in 
evolutionary  investiga- 
tions which  are  not 
entirely  speculative  in 
their  nature. 

It  will  be  convenient 
and  appropriate  at  this 
stage  to  consider  the 
root  in  the  herbaceous 
dicotyledons.  The 

Ranunculaceae  will  serve  here  as  an  excellent  exemplification  of  the 
conditions  in  plants  in  which  the  annual  herbaceous  habit  has 
brought  about  at  once  reduction  in  the  development  of  the  second- 
ary tissues  and  marked  modifications  in  their  topography.  Fig.  112 
illustrates  photographically  the  organization  of  a  root  in  the  case  of 
Actaea  alba.  The  cortical  tissues  are  somewhat  clearly  separated 
from  those  of  the  fibrovascular  system  by  an  endodermal  zone.  The 
central  region  of  the  root  is  distinguished  by  the  presence  of  four 
aggregates  of  phloem.  A  marked  feature  of  the  secondary  wood  is 
its  division  into  widely  separated  segments  by  broad  rays  of  the  com- 
pound type.  This  condition  is  extremely  common  in  roots  of  forms 
in  which  the  herbaceous  habit  has  become  strongly  developed. 


FIG.    112. — Transverse    section   of   a  root  of 
Actaea  alba. 


THE  ROOT 


157 


Fig.  113  shows  the  central  region  of  the  root  under  a  higher  degree  of 
magnification,  and  the  arrangements  described  above  now  become 
much  more  clearly  visible.  Another  structural  feature  now  for  the 
first  time  presents  itself — namely,  the  presence  of  groups  of  small- 
sized  elements  of  the  primary  xylem  in  alternation  with  the  more 


FIG.  113. — Part  of  same,  more  highly  magnified 


pronounced  radial  bands  of  secondary  wood  and  phloem.  The 
protoxylem  groups  are  incompletely  united  in  the  center,  and 
a  pithlike  cluster  of  parenchymatous  cells  is  consequently  found 
in  the  midst  of  the  primary  wood,  a  condition  quite  frequently 
present  both'  in  herbaceous  dicotyledons  and  generally  in  the 
monocotyledons.  In  the  woody  dicotyledons  and  in  the  conifers 
a  structure  comparable  to  the  pith  of  the  stem  is  ordinarily  absent 
in  the  root. 


158 


THE  ANATOMY  OF  WOODY  PLANTS 


In  the  monocotyledons  the  root  is  characterized  by  the  usual 
absence  of  any  indications  of  secondary  growth.  In  Fig.  114  is 
represented  the  transverse  view  of  the  root  of  the  carrion  plant, 
Smilax  herbacea.  The  root  hairs  are  very  conspicuous  (Fig.  115), 
and  underneath  the  layer  which  forms  them,  the  piliferous  layer,  is 
seen  a  zone  of  cells  which,  with  certain  exceptions,  possess  thickened 
walls.  Next  to  this  structure,  known  as  the  exodermis,  lies  the 

cortex,  which  terminates 
with  another  cellular 
limiting  membrane,  the 
endodermis.  The  center 
of  the  root  is  occupied 
by  the  central  cylinder 
and  the  pith.  The  for- 
mer consists  of  numer- 
ous and  distinctly 
alternating  clusters  of 
primary  xylem  and 
phloem  which,  as  is 
usual  in  roots,  occupy 
different  radii.  There 
is  no  indication  of 
secondary  thickening  in 
connection  with  either  the  xylem  or  the  phloem.  The  wood 
elements  are  smaller  on  the  outside  and  become  larger  as  they 
pass  inward.  Although  the  usual  gradation  in  size  is  found  to 
exist  in  the  elements  of  the  wood  in  monocotyledonous  roots,  the 
order  of  development  of  the  tracheary  cells  is  not  infrequently 
the  reverse  of  their  gradation  in  diameter.  In  other  words, 
the  internal  elements  of  a  tracheary  nature  are  completed  before 
those  lying  farther  outward  in  the  region  of  the  protoxylem.  This 
is  one  of  the  many  abnormalities  which  characterize  the  anatomical 
structure  of  this  important  group  of  the  angiosperms.  The  central 
region  of  the  root  is  occupied  by  a  well-marked  pith,  a  condition 
of  very  general  occurrence  in  the  group  under  discussion.  A 
similar  situation  in  regard  to  the  frequent  presence  of  medullary 
structures  is  found  in  the  case  of  herbaceous  dicotyledons.  It  seems 


FIG.  114. — Root  of  Smilax  herbacea 


THE  ROOT 


159 


certain  that  the  presence  of  a  pith  in  roots  is  not  a  primitive  feature, 
but  one  which  has  been  secondarily  acquired. 

In  the  mass  of  mono- 
cotyledonous  roots  there 
is  very  little  departure 
from  the  state  of  affairs 
found  in  the  illustrations 
shown  above.  At  most, 
the  modification  in  the 
central  cylinder  consists 
of  variations  in  the  num- 
ber of  groups  of  xylem 
and  phloem.  An  interest- 
ing complication  of  the 
piliferous  or  root-hair- 
forming  layer  is  fre- 
quently present  in  certain 
orchids  and  aroids.  Here 
the  normally  uniseriate 
piliferous  layer  becomes 
multiplied,  giving  rise  to 
a  white  spongy  substance 
covering  the  surface  of 
the  root  and  known  as 
the  velamen  or  veil.  This 
structure  is  peculiarly 
characteristic  of  the  aerial 
roots  of  tropical  epiphytes 
belonging  to  the  two 
orders  mentioned  above, 
although  sometimes  found 

in  a  condition  of  imper- 

,  j          ,  .  FIG.  115. — Part  of  transverse  section  of  root 

feet    development    in     oi  Smilax  herbacea, 

terrestrial  orchids  of  tem- 
perate climates.     The  general  relations  presented  by  the  velamen 
are  shown  in  Fig.  116.     Here  it  is  clear  that  the  structure  under 
consideration  lies  external  to  the  exodermis  and  is  consequently 


i6o 


THE  ANATOMY  OF  WOODY  PLANTS 


morphologically  equivalent  to  the  normally  single  piliferous  layer 
of  terrestrial  monocotyledonous  roots.  The  cells  of  the  velum 
have  banded  thickenings  in  their  walls  which  in  a  general  way 
simulate  those  found  in  certain  types  of  tracheids.  The  elements 
in  question  are  also  sometimes  porous,  so  that  water  enters  readily 
into  their  cavities  during  the  rainy 
season,  to  be  conserved  for  use  in  the 
following  period  of  drought. 

In  vascular  plants  in  general  the 
secondary  roots  are  related  in  a  definite 
fashion  to  the  primary  structures  of 
the  main  root.  In  the  greater  number 
of  cases  the  young  root  appears  as  a 
local  development  of  cells  on  the  cen- 
tral cylinder  of  the  main  root.  This 
cellular  proliferation  is  opposite  one  of 
the  clusters  of  protoxylem,  so  that  the 
arrangement  of  the  rootlets  is  predeter- 
mined by  the  organization  of  the  pri- 
mary structures  of  the  main  root.  In 
many  monocotyledons  the  indication  of 
a  developing  root  appears,  not  oppo- 
site one  of  the  protoxylem  groups,  but 
in  the  interval  between  two  of  these, 
thus  constituting  a  departure  from  the 
usual  topography.  It  is  evident  that 

the  secondary  roots  are  formed  endogenously  and  subsequently 
bore  their  way  outward.  This  internal  mode  of  origin  is 
characteristic  of  roots  and  rootlets,  the  only  exception  being 
found  in  the  case  of  the  primary  root  of  the  seedling.  Fre- 
quently roots  are  definitely  related  to  other  appendages,  such 
as  branches  and  leaves,  and  in  these  instances  originate  at  or 
near  the  node. 

All  roots,  with  the  exception  of  the  degenerate  ones  found  in 
certain  types  of  parasitism,  are  provided  with  a  protective  cover 
over  their  tender  apex,  and  this  is  known  as  the  root  cap  or  pile- 
orhiza.  This  structure  is  for  the  purpose  of  protecting  the  root 


FIG.  1 1 6. — Transverse  sec- 
tion of  an  orchid  root,  showing 
multiple  piliferous  layer  devel- 
oped as  a  velamen. 


THE  ROOT 


161 


as  it  forces  its  way  through  the  more  or  less  resistant  soil.  The 
tissues  of  the  root  usually  become  stiffened  at  an  early  stage  by 
the  rapid  thickening  of  the  walls  of  the  tracheary  elements  of  the 
primary  wood.  Rigidity  is  so  much  a  necessity  in  the  case  of  the 
wood  of  the  root  that  ringed  and  spiral  tracheids,  such  as  ordinarily 
characterize  the  first-formed  region  of  the  xylem  in  the  aerial  stem, 
are  conspicuous  by  their  absence  or  at  any  rate  by  a  scanty  degree 
of  development.  In  Fig.  117  are  represented  the  protoxylem  of 
the  stem  and  root  of  the  balsam  fir.  The  great  contrast  in  the 
development  of  the  extensible  ringed  and  spiral  tracheids  is  shown 


FIG.  117. — Longitudinal  view  of  the  primary  tracheids  of  the  root  and  stem  of 
the  balsam  fir,  showing  the  different  degrees  of  development  of  the  protoxylem  ele- 
ments in  the  two  organs. 

in  the  diagram.  It  is  obvious  that  ringed  and  spiral  sculpture  alone 
is  not  an  entirely  constant  character  of  the  first-formed  wood  in  roots. 
A  great  deal  of  morphological  importance  has  in  the  past  been 
attached  to  the  arrangements  of  cells  at  the  apices  of  the  roots. 
It  is  not  clear,  however,  that  inferences  drawn  from  such  data 
have  a  very  great  evolutionary  significance;  and  certainly  in  other 
organs  of  the  plant,  such  as,  for  example,  the  stem,  they  are  of 
very  slight  value  in  view  of  the  extremely  contradictory  results 
reached  as  a  consequence  of  adherence  to  this  criterion.  At  the 
present  time  the  organization  of  the  growing  point  in  plant  organs 
is  regarded  as  of  less  importance  than  the  histological  structure 
of  the  mature  parts  in  reaching  any  conclusions  as  to  equivalence 
and  course  of  evolution. 


CHAPTER  XIII 
THE  STEM 

In  the  older  types,  represented  by  the  vascular  cryptogams,  the 
general  organization  of  the  stem  presents  more  variety  in  ground 
plan  than  that  found  in  the  case  of  the  seed  plants.  In  these  earlier 
forms  the  fibro vascular  tissues  are  present  in  two  main  conditions. 
In  the  primitive  type  of  structure  the  xylem  is  a  solid  mass  which 
does  not  include  any  medullary  substance  or  pith.  This  variety 
is  represented  in  Fig.  118  and  is  known  as  protostelic.  The 
protostelic  condition  is  found  in  many  older  groups  and  is  of  almost 
universal  occurrence  in  the  most  conservative  of  all  the  organs  of 
the  plant,  the  root.  Where  the  mass  of  primary  xylem  is  smooth 
in  contour  and  does  not  show  on  its  surface  any  projecting  salients 
of  protoxylem,  the  phloem  forms  a  continuous  layer  surrounding 
it,  and  the  whole  fibrovascular  complex  so  organized  is  known  as 
concentric.  Usually  in  concentric  protosteles  the  protoxylem 
elements  are  situated  in  relation  to  the  general  organization  of  the 
xylem  in  the  manner  described  in  an  earlier  chapter  as  mesarch. 
Here  it  will  be  recalled  that  the  first-formed  elements  of  the 
wood  are  ultimately  completely  surrounded  by  those  of  later 
origin.  When,  by  reason  of  the  projecting  masses  of  protoxylem 
not  imbedded  in  the  main  mass  of  the  primary  wood  as  in 
the  mesarch  type,  there  are  more  or  less  deep  furrows  on  the  face 
of  the  xylem,  the  phloem  ceases  to  constitute  a  continuous  layer, 
but  is  present  in  isolated  strands  alternating  with  the  projecting 
angles  of  the  protoxylem,  and  the  stelar  structure  is  described  as 
radial  in  its  organization.  In  this  condition  the  development  of 
the  wood  is  usually  entirely  centripetal,  or  toward  the  center  of 
the  organ.  This  mode  of  organization  is  anatomically  described  as 
exarch,  because  the  protoxylem  elements  are  external  in  position 
to  all  the  later-formed  portion  of  the  wood. 

Another  primitive  type  of  structure  presented  by  the  fibro- 
vascular organization  of  the  stem  is  that  known  as  siphonostelic. 

162 


THE  STEM 


163 


Here  the  phloem  and  xylem,  instead  of  constituting  a  solid  mass 
without  any  parenchymatous  center  or  pith,  are  laid  down  in  the 
form  of  a  hollow  cylinder  inclosing  a  central  medulla  or  pith.  This 
type  of  organization  of  the  central  cylinder  or  stele  is  technically 
called  siphonostelic,  on  account  of  the  tubular  condition  present. 
Primitively  the  siphonostele  or  tubular  central  cylinder  seems  to 


FIG.  118. — Diagram  of  a  protostelic  stem 

have  been  organized  in  such  a  manner  that  phloem  clothed  its 
inner  surface  as  well  as  its  outer  one.  Further,  both  inwardly 
toward  the  pith  and  outwardly  toward  the  cortex  the  tissues  of 
the  central  cylinder  or  stele  are  clearly  delimited  from  those  of  the 
fundamental  system  by  a  well-marked  endodermal  layer.  The 
siphonostele  may  be  either  concentric  or  radial  in  its  organiza- 
tion, precisely  as  in  the  protostele,  the  general  situation  being 
identical  in  the  two  cases  except  for  the  presence  of  the  pith.  The 
essential  features  of  the  siphonostele  are  presented  in  Fig.  119. 
Here  may  be  seen  cortex  and  pith  marked  by  a  similarity  of 


1 64  THE  ANATOMY  OF  WOODY  PLANTS 

organization  and  separated  from  the  fibrovascular  category  of  tis- 
sues by  the  endodermis.  To  the  left  in  the  figure  the  siphonostele  is 
depicted  in  a  continuous  or  closed  condition,  while  on  the  right  it 
is  open  on  one  side.  The  opening  corresponds  to  the  giving  off 
of  the  trace  of  an  appendage,  which  in  this  instance  is  a  leaf.  A 
general  feature  of  the  tubular  or  siphonostelic  central  cylinder  in 


FIG.  119. — Diagram  of  a  siphonostelic  stem 

the  ferns  and  their  immediate  allies  is  the  appearance  of  openings 
in  the  tubular  central  cylinder  which  correspond  to  either  leaves  or 
branches.  These  openings  persist  for  a  longer  or  shorter  distance 
above  the  point  of  departure  of  the  fibrovascular  supply  to  the 
appendage  and  are  known  as  leaf  gaps  or  branch  gaps  according 
to  the  organ  involved.  Around  the  edges  of  these  gaps  the  endo- 
dermis of  the  inside  becomes  continuous  with  that  of  the  outside. 
Usually,  too,  the  phloem  of  the  inner  surface  of  the  tubular  stele 
joins  with  that  situated  on  the  exterior.  Naturally,  also,  the 
fundamental  tissue  of  the  outside,  commonly  known  as  the  cortex, 


THE  STEM  165 

becomes  reunited  with  that  situated  in  the  center  of  the  stele, 
designated  in  turn  as  the  medulla  or  pith. 

Fig.  119  depicts  a  siphonostelic  central  cylinder.  The  traces  of 
leaves  determine  the  presence  of  lacunae,  or  interruptions  in  the 
continuity  of  the  cylinder,  for  some  distance  above  their  points 
of  departure.  These  are  the  foliar  gaps  or,  in  case  they  are  related 
to  lateral  branches,  the  branch  gaps. 

The  protostelic  and  siphonostelic  conditions  of  stem  are  both 
subject  to  further  modification  which  may  now  be  discussed.  In 
the  protostelic  stem,  for  example,  the  phloem  may  more  or  less 
completely  disappear  on  one  side  of  the  stele,  a  situation  found  in 
the  stem  and  sometimes  even  in  the  root  of  certain  ferns.  This 
condition  is  known  as  collateral  and  is  extremely  rare  in  proto- 
stelic organs.  This  state  of  reduction  is,  however,  very  common 
in  the  siphonostele,  particularly  in  the  higher  forms  where  the 
tubular  type  of  central  cylinder  has  become  universal.  A  proper 
understanding  of  the  phenomena  of  reduction  presented  by  the 
siphonostele  is  advantageously  gained  by  the  consideration  of  allied 
forms  presenting  the  stelar  conditions  both  in  the  normal  and  in  the 
reduced  state.  Illustrations  of  this  nature  can  best  be  drawn  from 
the  lower  vascular  plants,  the  ferns  and  their  allies.  Fig.  120 
represents  on  the  left  a  siphonostelic  stem  in  which  internal  and 
external  phloem  are  both  well  developed,  and  likewise  the  endo- 
dermal  layers,  which  respectively  limit  these  from  pith  and  cortex. 
In  the  illustration  at  b  appears  a  stem  in  which  the  internal  phloem 
has  disappeared  on  most  of  the  interior  of  the  stele  and  is  found 
merely  on  the  margins  of  the  foliar  gap.  In  the  case  of  the  chan- 
neled leaf  trace,  subtending  the  foliar  gap,  in  contrast  to  the 
situation  presented  by  the  fibrovascular  system  of  the  stem,  the 
internal  phloem  is  as  well  marked  as  that  occurring  on  the  outer  or 
convex  surface.  This  is  in  accordance  with  a  general  principle 
to  be  more  definitely  elaborated  at  a  later  stage.  The  basal  region 
of  the  foliar  fibrovascular  strand  in  vascular  plants  is  found  in 
general  to  retain  the  more  primitive  type  of  organization.  In  the 
particular  instance  under  consideration  the  retention  of  internal 
phloem  in  the  leaf  trace  is  the  significant  feature  in  a  consider- 
able number  of  ferns  and  fern  allies.  It  may  be  stated  that  a, 


i66 


THE  ANATOMY  OF  WOODY  PLANTS 


universally  accepted  procedure  in  comparative  anatomy  is  to  judge 
the  former  organization  of  the  walls  of  the  stelar  tube  from 
pertinent  conditions  presented  by  the  leaf  traces  which  take  their 
origin  from  it.  In  other  words,  ancestral  conditions  which  have 
disappeared  in  the  stele  of  the  highly  progressive  and  easily  modi- 
fied stem  may  continue  almost  indefinitely  in  the  fibrovascular 


FIG.  120. — Diagram  showing  degeneracy  of  internal  phloem  and  endodermis 
in  the  siphonostelic  central  cylinder.     Explanation  in  the  text. 

structures  of  the  leaf.  The  great  value  of  this  principle  is 
quite  generally  admitted  in  the  case  of  the  gymnosperms,  but 
its  validity  for  the  fernlike  forms  is  unfortunately  not  so  uni- 
versally conceded  in  spite  of  cogent  logical  considerations  in  its 
favor.  At  c  is  represented  a  further  condition  of  reduction.  Here 
not  only  the  internal  phloem  lias  disappeared  in  the  stele,  but 
likewise  there  is  no  internal  endodermis,  although  both  are  present 
in  the  leaf  trace.  At  d  in  the  same  figure  is  seen  the  anatomical 


THE  STEM  167 

condition  presented  by  the  young  plant  or  sporeling.  Here  the 
tubular  central  cylinder  is  characterized  by  an  internal  as  well  as 
an  external  endodermal  layer.  The  general  principle  applicable  to 
this  situation  is  that  young  stages  frequently  perpetuate  conditions 
which  have  disappeared  in  the  adult.  It  is  apparently  clear  from 
commonly  accepted  canons  of  comparative  anatomy  which  will  be 
elaborated  in  a  subsequent  chapter  that  leaf  and  seedling  stem  fre- 
quently enable  us  to  reconstruct  the  course  of  evolution  in  the 
siphonostelic  cylinder. 

The  presence  of  internal  phloem  and  endodermis  has  been 
described  in  the  former  paragraph  as  a  primitive  feature  of  the 
earlier  types  with  the  siphonostelic  central  cylinder.  Other 
important  anatomical  structures  of  lower  or  cryptogamic  vascular 
plants  are  exarch  and  mesarch  primary  wood.  It  will  be  recalled 
that  the  term  exarch  has  reference  to  the  fact  that  the  protoxylem 
elements — that  is,  the  tracheary  tissues  which  are  first  laid  down — 
are  external  in  position,  and  that  the  subsequent  development  of 
elements  of  the  wood  is  centrad,  or  toward  the  center  of  the  organ. 
Likewise  in  the  case  of  mesarch  structure  the  situation  is  first 
characterized  by  the  exarch  condition,  but  after  a  certain  progress 
has  been  made  the  formation  of  new  elements  of  the  xylem  shifts 
to  the  external  aspect  of  the  protoxylem,  and  as  a  result  we  find 
the  first-formed  elements  more  or  less  completely  surrounded  by 
those  of  later  origin.  Later  in  geological  time  and  in  still  higher 
types  the  mesarch  condition  in  turn  gives  place  to  the  endarch 
by  the  disappearance  of  the  centrad  or  centripetal  wood.  This 
situation  can  best  be  illustrated  by  a  diagram.  In  Fig.  121  at  a 
is  shown  the  exarch  condition  of  development  of  the  structures  of 
the  primary  wood.  The  small-sized  elements,  normally  ringed  or 
spiral,  are  outside  the  later-formed  scalariform  or  pitted  tracheids. 
In  b  is  represented  the  mesarch  condition,  in  which  the  development 
is  at  first  centrad,  but  later  becomes  peripherad,  so  that  the  pro- 
toxylem comes  to  occupy  a  central  position.  In  c  is  shown  the 
endarch  condition  where  the  seriation  from  the  protoxylem  is 
no  longer  centrad  but  has  become  entirely  peripherad  or  centrifu- 
gal. The  condition  in  a  characterizes  the  organization  of  the 
primary  xylem  in  the  stem  of  the  lycopods  and  their  allies,  and  is 


i68 


THE  ANATOMY  OF  WOODY  PLANTS 


found  without  exception  in  the  roots  of  all  vascular  plants.  The 
situation  diagrammatically  delineated  in  b  prevails  in  the  stem 
organs  of  the  ferns  and  their  allies,  the  lower  gymnosperms.  In 
the  higher  gymnosperms  and  the  angiosperms  the  axial  organs 
exemplify  almost  exclusively,  in  living  types  at  any  rate,  the 
endarch  condition  diagrammatically  represented  in  c.  As  might  be 
expected,  the  primitive  occurrence  of  exarch  and  mesarch  conditions 


B 


FIG.  121. — Diagram  to  indicate  the  relations  of  protoxylem  to  metaxylem  in 
vascular  plants.  Explanation  in  the  text. 

in  the  case  of  the  stem  can  frequently  be  inferred  from  the  anatom- 
ical situation  presented  by  the  leaves.  The  universal  presence  of 
the  exarch  type  of  primary  wood  in  the  root  furnishes  good  evidence 
that  this  was  the  original  type  of  organization  and  development 
of  the  primary  wood  in  the  vascular  series. 

It  has  been  made  evident  in  an  earlier  chapter  that,  in  addition 
to  the  primary  organization  of  the  wood  consisting  of  tracheary  and 
parenchymatous  elements  arranged  in  irregular  order,  there  is 
often  present,  particularly  in  arboreal  types  and  perennials,  ancient 


THE  STEM  169 

and  modern,  a  subsequent  ligneous  development  made  up  of  cells 
regularly  seriate  radially  and  including  masses  of  radial  parenchyma. 
This  regularized  and  later-appearing  xylem  is  known  as  the  second- 
ary wood.  In  older  types  the  primary  wood  is  distinct  from  the 
secondary  ligneous  structure,  and  its  conspicuousness  under  these 
conditions  is  due  largely  to  the  more  abundant  development  of  the 
primary  elements  and  their  frequent  exarch  or  mesarch  configura- 
tion. In  the  higher  gymnosperms  and  in  the  angiosperms  they 
can  be  recognized  only  with  difficulty  except  in  the  root.  This 
situation  is  the  result  of  the  slight  development  of  the  primary 
elements  and  of  the  fact  that  they  are  in  series  with,  and  formed  in, 
the  same  direction  as  the  secondary  wood. 

It  will  be  apparent  from  the  statements  made  in  the  preceding 
paragraphs  that  the  organization  of  the  stem  is  characterized  by 
a  considerable  degree  of  variety  as  regards  the  general  topography 
and  microscopic  organization  of  the  primary  structures.  When 
the  special  consideration  of  the  ferns  and  their  allies  is  reached  in 
later  chapters,  it  will  be  clear  that  the  possible  complexities  of  the 
gross  fibrovascular  structures  have  been  by  no  means  exhausted. 

It  is  now  convenient  to  turn  our  attention  to  the  secondary 
fibrovascular  structures  of  the  stem.  In  the  older  vascular 
plants  the  secondary  wood  was  extremely  simple  in  its  organi- 
zation and  consisted  merely  of  radial  parenchyma  and  longitudinal 
tracheary  elements.  The  radial  parenchyma  in  the  lowest  forms 
was  definitely  separated  from  the  pith,  as  is  well  illustrated  in 
Fig.  122,  by  the  presence  of  a  clearly  developed  zone  of  primary 
wood.  With  the  progress  of  geological  time  came  a  progressive 
reduction  of  the  primary  wood  and  important  modifications  in 
the  structure  of  the  secondary  woody  cylinder.  The  gradual 
reduction  of  the  primary  structures  of  the  woody  cylinder  has 
involved  interesting  conditions  which  must  now  occupy  our 
attention.  The  progressive  reduction  of  the  region  of  the  wood 
known  as  primary  brought  with  it  certain  important  topographical 
changes.  The  primary  wood  was  at  first  continuous  and  in  most 
instances  constituted  in  the  stem  a  siphonostele  interrupted  only 
by  the  passing  off  of  the  traces  of  branches  or  leaves.  The  breaks 
in  the  continuity  of  the  primary  cylinder  thus  determined  are 


170 


THE  ANATOMY  OF  WOODY  PLANTS 


known  as  leaf  or  branch  gaps,  as  the  case  may  be.  In  the  lower 
forms,  with  super-addition  of  secondary  to  primary  woody  struc- 
tures, the  primary  cylinder  of  the  siphonostele  still  maintained  to 
a  large  degree  its  individuality.  This  is,  for  example,  true  in 

Fig.  i22a  representing  the  organiza- 
tion of  a  stem  in  a  lepidodendrid, 
an  arboreal  club  moss  of  the  Pale- 
ozoic age.  The  lepidodendroid 
arboreal  club  mosses  include  forms 
referred  to  the  general  cognomen 
of  Sigillaria.  In  lycopods  of  this 
type,  particularly  in  the  Permian 
age  and  toward  the  end  of  their 
term  of  existence  as  an  element  of 
our  earth's  flora,  the  primary  wood 
became  much  reduced  in  amount. 


FIG.  122 


FIG.  122  a,  b,  and  c. — Diagrams  to  illustrate  the  effect  of  the  degeneracy  of  the 
primary  wood  on  the  development  of  the  secondary  xylem. 

Not  only  did  this  situation  express  itself  in  the  thinning  down  of 
the  primary  woody  cylinder,  but  there  was  also  present  a  lack  of 
continuity  due  to  the  complete  elimination  of  tracheids  in  certain 
segments.  This  phase  is  revealed  in  b.  Here  the  primary  wood 
constituting  a  continuous  cylinder  in  a  is  interrupted  by  intervals 
resulting  from  the  scanty  development  of  the  primary  xylem. 
The  gaps  between  the  resulting  slender  isolated  strands  are  per- 


THE  STEM  171 

petuated  in  the  early  development  of  the  secondary  organization, 
since  the  radially  disposed  elements  make  their  first  appearance 
opposite  the  clusters  of  primary  wood,  and  only  as  the  second- 
ary segments  widen  out  later  do  they  finally  bridge  over  the 
intervals  in  the  continuity  of  the  cylinder.  On  the  upper  side 
a  branch  causes  a  still  more  prominent  hiatus  in  the  inner  region 
of  the  cylinder.  The  condition  in  a  calamitean  stem  may  next 
engage  our  attention.  Here  the  primary  wood  is  much  less  well 
developed  even  than  in  the  later  Sigillariae,  and  only  in  the 
earliest  representatives  of  the  genus  does  it  show  an  indication 
of  the  ancestral  centripetal  type  of  development.  The  task  of 
bridging  over  the  primary  intervals  by  secondary  growth  is  cor- 
respondingly greater  than  in  the  sigillarian  stem,  and  the  bays 
extending  into  the  secondary  wood  proportionately  deeper  and 
wider.  In  the  upper  region  of  the  cylinder  is  figured  a  siphonostelic 
branch  which  is  responsible  for  a  wide  hiatus  in  the  secondary 
cylinder.  An  inspection  of  a,  b,  and  c  makes  it  clear  that  the 
progressive  degeneracy  of  the  primary  wood  as  well  as  the  depar- 
ture of  traces  belonging  to  the  branches  causes  interruptions 
in  the  continuity  of  the  secondary  wood.  These  are  rilled  by  soft 
tissues  continuous  with  the  parenchyma  of  the  pith.  The  second- 
ary cylinder  is  characterized,  as  has  been  demonstrated  in  the 
diagrams,  by  bands  of  radial  parenchyma  which  are  often  wrongly 
called  medullary  rays.  That  they  cannot  receive  this  appellation  , 
with  any  accuracy  will  be  apparent  from  a  comparison  of  the 
conditions  presented  by  a,  b,  and  c.  In  a  the  rays  in  no  case  reach 
the  pith.  In  b  the  radial  parenchyma  is  clearly  continuous  with 
the  soft  tissues  of  the  pith  in  the  intervals  between  the  isolated 
segments  of  primary  wood,  while  in  those  regions  of  the  secondary 
cylinder  subtended  by  primary  xylem,  which  of  course  represent 
the  primitive  topographical  relation,  no  such  continuity  is  possible. 
In  c,  by  the  still  further  reduction  of  the  primary  structures  of  the 
wood,  a  much  larger  number  of  bands  of  radial  parenchyma  appear 
to  abut  on  the  pith.  It  is  clear  from  the  conditions  outlined  in  the 
diagram  under  discussion  that  it  is  entirely  inappropriate  to  de-  \ 
scribe  the  radial  storage  tissues  of  the  secondary  wood  as  medullary 
rays.  A  satisfactory  appellation  for  them  is  simply  wood  rays,  and 


172 


THE  ANATOMY  OF  WOODY  PLANTS 


this  involves  no  erroneous  hypothesis  of  their  relation  to  the  pith. 
It  is  therefore  necessary  to  distinguish  carefully  between  wood  rays 
and  gaps  or  discontinuities  in  the  woody  cylinder  originating  as  a 
result  of  conditions  described  above.  It  seems  quite  evident  that 
the  failure  to  realize  this  distinction  invalidates  investigations  on 
the  woody  cylinder  involving  the  confusion  of  thought  elucidated 

by  the  items  in 
the  accompanying 
figure. 

The  last  para- 
graph will  have  con- 
vinced the  reader 
that  radial  bands  of 
parenchyma,  formed 
as  a  result  of  cam- 
bial  activity,  can- 
not accurately  be 
described  as  medul- 
lary rays.  We  are 
now  in  a  position  to 
consider  the  situa- 
tion presented  by 
the  stem  of  peren- 
nial seed  plants  of 
gymnospermous  af- 
finities. Fig.  123  reproduces  part  of  a  transverse  section  of  a  stem 
of  the  white  spruce,  Picea  canadensis.  The  woody  cylinder  is 
characterized  by  numerous  radial  bands  of  parenchyma,  appar- 
ently in  every  instance  taking  their  origin  from  the  medulla  or 
pith.  Further,  there  are  present  broad  outwardly  directed  bays 
from  the  medullary  region  which  represent  the  gaps  correspond- 
ing to  outgoing  leaves  (for  in  the  gymnosperms,  in  contrast  to 
the  lycopods  and  their  allies,  the  leaves  are  related  to  foliar  gaps). 
A  superficial  examination  of  the  figure  would  justify  the  applica- 
tion of  the  term  medullary  ray  both  to  the  narrow  radial  bands  of 
parenchyma  and  to  the  broader  bays  extending  from  the  pith. 
The  considerations  advanced  in  the  last  paragraph  make  it  clear, 


FIG.  123. — Stem  of  Picea,  showing  apparent  relation 
of  wood  rays  to  pith. 


THE  STEM 


173 


however,  that  this  is  an  inaccurate  interpretation.  The  justice  of 
the  criticism  here  advanced  is,  moreover,  evidenced  by  reference 
to  the  very  conserva- 
tive structures  of  the 
root.  Fig.  1 24  illus- 
trates the  central 
region  of  this  organ 
in  the  same  species 
of  spruce.  The 
primary  xylem  is 
pronounced  and  dis- 
tinctly centripetal  in 
the  order  of  develop- 
ment of  its  elements. 
There  is  no  medulla 
present,  and  the  rays 
of  the  secondary 
wood  consequently 


FIG.  124. — Root  of  Picea,  showing  primitive  condition 


not  only  have  no  re- 
lation to  a  medullary 
region,  but  also  end  in  the  vicinity  of  the  primary  wood.  It  is 

clear  that  the  radial 
parenchyma  of  the 
root  cannot  come 
under  the  term 
medullary  ray,  and  it 
will  now  be  obvious  to 
the  reader  that  in  no 
case  can  the  radial 
bands  of  storage  tis- 
sue in  the  secondary- 
wood  be  accurately 
called  medullary  rays. 
They  are  properly  de- 
scribed as  wood  ravs. 
It  will  not  be 
FIG.  125.— Stem  of  Ephedra  gerardiana  necessary  to  consider 


174 


THE  ANATOMY  OF  WOODY  PLANTS 


any  other  lower  seed  plants  in  the  present  connection.     The  Gne- 
tales  and  angiosperms  present  the  next  significant  modification  of 

structure  in  the 
stem.  Fig.  125 
illustrates  the 
organization  of  the 
axis  of  Ephedra 
gerardiana.  Obvi- 
ously there  are 
numerous  broad 
bands  of  radial  pa- 
renchyma present. 
Another  feature  of 
interest  is  the  pres- 
ence of  true  vessels 
in  the  cylinder  of 
secondary  wood. 
Broad  rays  of  the 

FIG.  126. — Tangential  section  of  wood  of  Ephedra      type    found    in 
species,  showing  presence  of  large  rays.  Ephedra  and  in  the 

angiosperms  are 
definitely  corre- 
lated with  the  ap- 
pearance of  vessels. 
A  tangential  sec- 
tion (Fig.  126) 
through  the  wood 
of  Ephedra  reveals 
the  true  nature  of 
the  rays  in  the 
Gne tales.  Fig.  127 
illustrates  the 
organization  of  an 
individual  large  ray 
in  the  outer  region 
of  the  woody  cylin-  FIG  I27._Portion  o{  large  ray  highly  magnified> 

aer.     The  ray  IS     showing  presence  of  tracheary  elements  in  ray  substance. 


THE  STEM 


175 


plainly  not  of  homogeneous  organization,  but  contains  in  its  sub- 
stance, in  addition  to  true  radial  parenchyma,  large  quantities  of 
fibrous  elements  and  even  some  vessels.  The  fibrous  elements  when 
studied  under  higher  magnification  than  that  of  the  figure  often  show 
a  gradual  transition  to  parenchyma.  By  referring  to  chapter  vi 
it  will  be  seen  that  the  large  ray  of  Ephedra  comes  under  the  heading 
of  aggregate  ray,  described  in  that  chapter,  since  it  is  a  composite 
structure  made  up 
partially  of  true 
storage  cells  and 
partially  of  longi- 
tudinal elements  of 
the  wood  in  process 
of  parenchymatous 
transformation. 
Fig.  128,  which 
shows  the  ray  of 
Ephedra  in  its  nar- 
rower condition 
nearer  the  pith, 
supports  the  view 
of  the  nature  of  the 
large  ray  arrived 
at  as  a  consequence 
of  an  examination 
of  its  organization 

in  the  outer  region  of  the  wood.  In  its  earlier  stage  of  develop- 
ment it  contains  equally  clearly  a  mixture  of  storage  parenchyma 
and  of  fibers  more  or  less  completely  transformed  into  cells 
resembling  the  ordinary  elements  of  the  rays.  It  is  obvious 
in  Ephedra  (and  this  statement  holds  of  the  Gnetales  generally) 
that  the  higher  organization  of  the  water-conducting  elements  of 
the  wood  carries  with  it  a  more  abundant  provision  for  the  storage 
of  food  products  elaborated  by  the  leaves.  Of  course  the  stage 
of  evolution  here  attained  is  quite  impossible  in  the  case  of  a  more 
primitive  type  of  wood  in  which  longitudinal  parenchyma  either 
has  not  yet  made  its  appearance  at  all  or  has  progressed  to  such 


FIG.  128. — Aggregate  ray  of  Ephedra  species  in  region 
near  the  pith. 


i76 


THE  ANATOMY  OF  WOODY  PLANTS 


a  slight  degree  that  it  does  not  constitute  an  important  feature 
of  organization.  With  the  strong  development  of  longitudinal 
storage  cells,  as  has  been  explained  in  an  earlier  chapter,  owing  to 
the  septation  of  elements  originally  destined  to  become  tracheids, 
a  co-ordination  between  these  and  the  earlier  evolved  radial 
parenchyma  provides  possibilities  in  connection  with  storage  quite 
adequate  to  accommodate  the  output  of  the  more  efficient  and 

hygrophilous  leaf  of 
modern  floras.  The 
aggregate  ray  is  evi- 
dently the  result  of  the 
correlation  of  radial  and 
longitudinal  storage  de- 
vices. Its  appearance 
is  a  phenomenon  of 
prime  biological  impor- 
tance and  is  intimately 
related  to  the  origin  of 
the  herbaceous  type  in 
the  angiosperms  on  the 
one  hand  and  the  evolu- 
tion of  the  highest 
vertebrates,  the  warm- 
blooded mammals,  on 
the  other.  In  Gnetum, 

the  highest  genus  of  the  Gnetales,  the  condition  of  aggregation 
is  no  longer  prominent,  but  has  been  superseded  by  the  compound 
type  of  ray. 

The  next  significant  forms  to  occupy  our  attention  are  the  woody 
dicotyledons.  These  are  best  introduced  in  connection  with  the 
alder  and  the  oak.  In  Fig.  1 29  appears  a  photograph  of  a  branch 
of  Alnusjaponica  several  years  old.  The  central  pith  is  triangular, 
corresponding  to  the  one-over-three  phyllotaxy  of  the  genus.  The 
annual  rings  which  surround  the  pith  are  characterized  by  the 
presence  of  conspicuous  broad  rays,  of  which  the  largest  and  most 
striking  extend  from  the  sides  of  the  longest  angle  of  the  triangular 
pith.  These  most  conspicuous  radial  structures  are  related  to  the 


FIG.  129. — Transverse  section  of  the  stem  of 
Alnusjaponica,  showing  presence  of  aggregate  rays. 


THE  STEM 


177 


two  lateral  traces  of  a  leaf  and  may  consequently  be  appropriately 
called  leaf  or  foliar  rays.  The  less  conspicuous  rays  in  other  radii  of 
the  stem  are  in  relation  to  other  traces  of  other  leaves  lower  or  higher 
in  the  stem.  In  the  following  illustration  (Fig.  130)  a  portion  of  the 
stem  represented  in  Fig.  129  is  shown.  The  greater  magnification 
makes  it  apparent  that  the  broad  radial  band  related  to  the  out- 
going leaf  trace  is  in  reality  an  aggregation  of  rays  and  not  a  simple 
structure.  It  is,  in 
fact,  largely  com- 
posed of  fibers  as 
well  as  of  radial 
parenchyma,  to 
the  exclusion  of  the 
vessels  which  form 
a  prominent  fea- 
ture of  the  remain- 
ing organization  of 
the  wood.  In  the 
case  of  the  large 
ray  figured  it  is 
clear  that  conspic- 
uous depressions 
mark  the  surface  of 
the  annual  ring 
where  the  broad 
radial  band  crosses 
it.  In  Fig.  131  the  same  ray  appears  in  the  vertical  section  of  the 
wood.  It  is  now  evident  that  the  structure  in  question  consists 
of  an  aggregation  of  rather  small  rays  separated  from  one  another* 
by  fibers  and  including  in  their  midst  the  transverse  section 
of  a  foliar  strand  or  leaf  trace.  It  will  be  obvious  from  the 
various  figures  of  the  stem  of  the  alder  that  in  this  genus  there  are 
groupings  of  rays  in  relation  to  the  foliar  traces,  and  that  these 
storage  bands  on  account  of  their  topographical  and  physiological 
relations  may  appropriately  be  designated  foliar  rays. 

It  will  now  be  convenient  to  refer  to  the  conditions  found  in 
the  case  of  the  oak.     Fig.  132  illustrates  a  transverse  section  of 


FIG.  130. — Part  of  transverse  section  of  stem  of  Alntts 
japonica,  showing  aggregate  ray  in  relation  to  a  leaf,  more 
highly  magnified. 


i78 


THE  ANATOMY  OF  WOODY  PLANTS 


the  wood  of  the  red  oak,  Quercus  rubra.     Large  rays  are  plainly 
present  which  differ  from  those  found  in  the  alder  by  the  fact 

that,  instead  of 
being  composed  of 
a  mixture  of 
smaller  rays  and 
separating  fibers, 
they  are  consti- 
tuted entirely  of 
parenchymatous 
elements.  In  spite 
of  this  difference  in 
the  organization  of 
the  broad  bands  of 
radial  parenchyma 
they  are  correlated 
with  the  same  dips 

in  the  annual  rings 
FIG.  i3i.-Tangential  section  of  stem  of  Alnus      found  jn  ^  aWer 
japonica,  showing  leaf  trace  imbedded  in  the  aggregate  ••_«•, 

ray  to  which  it  is  related.  as  described  in  the 


foregoing  paragraph. 
In  Fig.  133,  p.  180, 
is  shown  a  trans- 
verse section  of  a 
branch  of  the  velvet 
oak.  Here  there  are 
five  pairs  of  rays 
corresponding  to  the 
five  faces  of  the  five- 
angled  pith,  which 
by  its  configuration 
indicates  the  phyllo- 
taxy  precisely  as 
does  the  triangular 
medulla  in  the  alder. 
In  the  oak  there  are 


FIG.  132. — Transverse  section  of  the  wood  of  the  red  oak 


THE  STEM  179 

two  important  lateral  traces  in  relation  to  each  leaf,  and  less 
well-developed  central  ones.  The  most  distinct  foliar  rays  are 
those  formed  in  relation  to  the  lateral  traces;  since  there  are 
two  of  these  for  each  leaf  and  five  leaves  in  a  cycle,  there  are  ten 
conspicuous  rays  in  the  woody  cylinder  of  the  stem.  An  inspection 
of  the  figure  makes  it  clear  that  the  rays  are  related  in  approximated 
pairs,  and  that  in  the  narrower  intervals  intervening  between  the 
foliar  rays  the  woody  cylinder  is  depressed.  These  depressed 
segments  are  five  in  number,  corresponding  to  the  five  pairs  of 
propinquitous  large  rays.  The  natural  explanation  of  the  depres- 
sion is  that  it  is  the  result  of  the  local  effect  of  the  large  rays  on  the 
rate  of  growth  of  the  cylinder.  It  has  been  made  clear  above  that 
wherever  a  large  ray  crosses  an  annual  ring  a  depressing  influence 
manifests  itself,  resulting  in  a  corresponding  dip  in  the  surface  of 
the  yearly  zone  of  growth.  Obviously  if  two  large  rays  occur 
close  to  one  another  they  will  be  likely  to  exercise  a  depressing 
effect  on  the  region  of  the  annual  rings  lying  between  them.  That 
this  is  the  real  explanation  of  the  depressed  segments  of  the  stem 
of  the  oak  and  the  stems  of  a  similar  type  of  organization  is  shown 
by  the  fact  that,  where  the  broad  rays  are  equidistant,  as,  for 
example,  in  the  grapevine,  the  segments  fail  to  become  depressed. 
On  the  other  hand,  where  broad  rays  happen  to  be  absent  for  any 
cause  in  the  branches  of  the  oak  the  depression  is  likewise  absent. 
A  further  elucidation  of  the  situation  is  furnished  by  the  ranun- 
culaceous  genus  Clematis.  Here  in  species  with  approximated 
broad  rays  there  are  depressed  intervening  segments,  while  in  the 
few  species  where  the  broad  rays  are  equidistant  no  such  depressions 
occur. 

The  existence  of  depressed  segments  (Fig.  133)  in  the  stems  of 
woody  dicotyledons  with  large  rays  has  been  made  the  basis  of 
an  erroneous  and  historically  incongruous  hypothesis  of  the  evo- 
lution of  the  woody  type.  This  misconception  originated  with 
the  Prussian  botanist  Sanio  in  the  nineteenth  century,  and,  elabo- 
rated in  clear  diagrams  in  Sachs's  classic  textbook,  has  become  uni- 
versal in  the  pedagogical  literature  of  botany.  It  is  considered  that 
the  primordial  condition  of  the  woody  cylinder  is  that  of  numeri- 
cally varying  separate  bundles.  These  strands  of  fibrovascular 


i8o 


THE  ANATOMY  OF  WOODY  PLANTS 


tissues  are  imagined  to  have  been  originally  quite  distinct  from  one 
another  and  to  have  been  linked  up  by  the  formation  of  woody 
commissures  organized  entirely  of  secondary  xylem  and  laid  down 
by  the  activity  of  the  so-called  interfascicular  cambium.  The 
narrower  and  more  depressed  segments  are  supposed  to  owe  their 
peculiarities  to  their  late  and  entirely  secondary  origin.  The  five 
outstanding  segments  of  wood  in  the  oak  and  similar  forms  have 

been  accordingly  dubbed 
the  fascicular  wood,  and 
the  five  intervening  de- 
pressed segments  the 
interfascicular  wood. 
An  unfortunate  situa- 
tion often  encountered 
by  this  hypothesis  is  the 
fact  that  primary  wood 
is  frequently  as  well 
developed  on  the  inner 
surface  of  the  depressed 
segments  as  on  that  of 
the  outstanding  ones. 
The  depression,  as  has 
been  pointed  out  above, 
is  susceptible  of  an 
entirely  different 
explanation — namely,  as 

the  result  of  the  local  inhibiting  influence  of  approximated  broad 
rays  on  the  rate  of  growth  of  the  annual  ring.  In  effect,  moreover, 
the  hypothesis  of  Sanio  and  Sachs  derives  woody  from  herbaceous 
forms,  a  conclusion  entirely  at  variance  with  the  paleontological  his- 
tory of  plants.  It  is  clear  that  the  woody  forms  have  preceded 
herbaceous  ones  in  all  the  main  series  of  vascular  plants.  A  single 
illustration  will  serve  in  the  present  connection.  Our  somewhat 
herbaceous  existing  lycopods  and  Equiseta  are  certainly  known  to 
have  come  from  ancestral  forms  which  possessed  so  conspicuously 
the  arboreal  and  perennial  habit  that  for  many  years  a  controversy 
raged  as  to  their  affinities.  The  majority  of  paleobotanists  for  a 


FIG.  133. — Transverse  section  of  a  twig  of  the 
velvet  oak  (Quercus  velulina)  showing  five  pairs  of 
foliar  rays. 


THE  STEM 


181 


long  period  considered  them  as  belonging  to  the  seed  plants,  since 
all  the  types  of  the  present  age  which  possess  marked  secondary 
growth  are  seed  plants  of  gymnospermous  or  angiospermous  affini- 
ties. The  situation  in  regard  to  the  relation  of  sequence  between 
herbaceous  and  woody  types  might  be  almost  endlessly  illustrated, 
but  the  examples  cited  will  serve  for  the  present. 

It  has  been  assumed  in  the  references  to  the  large  rays  in  the 
alder  and  the  oak 
that,  although 
differing  from  one 
another  in  organiza- 
tion (in  one  case 
aggregate  and  in  the 
other  compound), 
they  have  the  same 
morphological  ex- 
planation. It  is 
clear  from  the  de- 
scriptions supplied 
in  the  present  and  a 
foregoing  chapter 
that  they  are  simi- 
larly related  to  the 
appendages.  It  is 
now  apposite  to 
establish  the  fact 

that  the  two  types  are  related  to  one  another  in  the  particular 
forms  under  discussion  in  the  present  connection.  Fig.  134  illus- 
trates the  organization  of  the  large  ray  in  the  wood  of  the  root 
of  the  red  oak  (Quercus  rubra)  as  seen  in  transverse  section.  It 
is  quite  obvious  that  the  structure  differs  from  that  found  in 
the  stem  as  exemplified  in  Fig.  135  by  the  fact  that  the  ray  is 
not  homogeneous  in  its  organization,  but  has  numbers  of  fibers 
intermingled  with  its  parenchymatous  elements.  In  other  words, 
the  large  mass  of  radial  parenchyma  under  discussion  must  be 
considered  as  being  of  aggregate  organization  and  not  as  belong- 
ing to  the  category  of  rays  defined  in  an  earlier  chapter  as 


FIG.  134.— Transverse  section  of  large  ray  in  root  of 

Quercus  rubra. 


182 


THE  ANATOMY  OF  WOODY  PLANTS 


compound.  In  Fig.  136  the  longitudinal  view  of  the  root  in  the  red 
oak  is  shown.  It  is  now  more  than  ever  evident  that  the  compound 
condition  of  the  rays  characteristic  of  the  stem  of  the  oak,  at  least 
so  far  as  the  red  oak  is  typical  of  the  genus,  is  not  present  in  the  roots, 
but  that  in  the  latter  organ  the  broad  bands  of  radial  storage 
elements  belong  to  the  type  denned  as  aggregate.  In  the  case 
of  the  root,  in  fact,  one  finds  for  many  years  of  development  that 

the  more  prominent 
radial  structures  are 
aggregate,  a  condi- 
tion which  gives 
place  in  old  and 
thick  axes  to  the 
compound  type  of 
organization.  In 
the  seedling  stem 
or  in  branches  de- 
rived as  the  result 
of  injuries  from  the 
lowest  region  of  the 
main  trunk,  the 
aggregate  ray  occurs 
and  persists  often 
for  a  long  period.  In 
anticipation  of  cer- 
tain general  prin- 
ciples of  comparative  anatomy  to  be  set  forth  in  detail  in  a  later 
chapter  of  this  work  it  may  be  stated  that  it  is  apparent  in  the 
case  of  the  oak  that  the  aggregate  condition  of  the  rays  has 
preceded  that  defined  as  compound,  and,  in  fact,  that  the  aggre- 
gate condition  is  the  forerunner  of  the  compound  ray.  It  is 
accordingly  clear  that  as  regards  both  the  nature  of  the  large 
rays  and  the  relationship  which  they  bear  to  the  appendages  the 
alder  and  the  oak  are  substantially  in  a  similar  condition.  In 
other  words,  the  question  of  large  storage  rays  is  one  susceptible 
of  elucidation  in  accordance  with  well-defined  evolutionary  and 
physiological  principles. 


FIG.  135. — Transverse  section  of  large  ray  in  stem  of 
Quercus  rubra. 


THE  STEM 


183 


In  the  earlier  chapter  dealing  with  the  various  types  of  rays, 
particularly  in  those  paragraphs  concerning  the  radial  parenchyma 
of  the  higher  seed  plants,  it  has  been  shown  that  the  great  mass  of 
arboreal  dicotyledons  is  characterized,  not  by  extremely  large 
rays  in  marked  contrast  to  radial  parenchymatous  strands  of  the 
uniseriate  type,  but  by  rays  of  more  moderate  dimensions  scattered 
throughout  the  wood.  Obviously,  just  as  the  compound  type  of 
radial  parenchyma 
has  taken  its  origin 
from  the  fusion  of 
the  units  of  the  con- 
geries of  rays  known 
as  aggregate,  so  the 
diffuse  condition  of 
rays  characteristic 
of  the  dicotyledo- 
nous forest  trees  in 
general  has  taken  its 
origin  by  the  pro- 
gressive divergence 
of  the  original  com- 
ponents of  aggregate 
rays  in  the  outer 
annual  rings  of  older 
stems.  The  phe-  Quercus rubra. 
nomenon  of  diver- 
gence can  actually  be  seen  with  clearness  in  the  genus  Casuarina, 
while  in  other  cases  rays  of  the  aggregate  type  persist  in  the  root 
or  may  be  recalled  as  the  result  of  injury  even  when  they  are  not 
found  in  the  adult  stem.  It  may  accordingly  be  stated  for  the 
woody  dicotyledons  in  general  that  in  all  there  was  primitively 
present  the  category  of  ray  known  as  aggregate.  As  a  result  of 
the  fusion  of  the  elements  of  such  rays,  compound  rays  have 
made  their  appearance.  Further,  as  a  consequence  of  the  diver- 
gence of  the  members  of  the  aggregations,  the  diffuse  con- 
dition is  reached.  The  former  state  of  rays  is  extremely  rare 
in  forest  trees  of  angiospermous  affinities,  while  the  latter 


FIG.  136. — Tangential  section  of  large  ray  in  root  of 


1 84  THE  ANATOMY  OF  WOODY  PLANTS 

prevails  almost  universally  in  the  organization  of  their  woody 
cylinder. 

It  will  now  be  advantageous  to  consider  those  angiospermous 
stems  which  are  united  in  habits  under  the  forms  known  as  vines 
and  herbs.  In  this  connection  it  will  be  well  to  restrict  our  atten- 
tion to  the  typical  representatives  of  the  two  groups,  since  plants 
which  are  not  clearly  marked  either  as  herbaceous  forms  or  as 


FIG.  137. — Diagram  of  the  organization  of  the  stem  in  Leea,  a  shrubby  tropical 
representation  of  the  Vitaceae. 

vines  are  of  very  little  importance  in  the  present  connection.  A 
general  feature  of  the  forms  included  under  the  two  headings  here 
discussed  is  the  presence  of  large  rays  of  the  oak  type.  This 
situation  has  long  been  recognized  in  the  case  of  the  particular 
modifications  of  the  stem  to  be  elucidated  in  the  present  and  in  a 
following  paragraph.  The  vine  type  may  well  stand  first,  as  it  is 
nearer  the  ordinary  woody  perennial  condition  of  organization 
than  that  found  in  herbs.  Fig.  137  illustrates  the  organiza- 
tion of  the  woody  cylinder  of  Leea,  a  shrubby  tropical  genus 
belonging  to  the  Vitaceae.  Here  we  find  very  obviously  both 


THE  STEM 


185 


compound  and  primitive  rays.  In  shrubby  representatives  of 
the  genus  Vitis  occurring  in  the  southwest  region  of  the  United 
States  there  is  sometimes  present  the  same  mode  of  organization 
of  the  rays  as  characterizes  the  stem  of  Leea  figured  above.  In 
more  northern  species  of  Vitis,  however,  a  very  different  situa- 
tion presents  itself — namely,  that  found  in  Fig.  138.  Here  the 
only  rays  present  are  of  the  compound  type.  An  interesting 


FIG.  138.— Diagram  of  the  organization  of  the  stem  in  a  northern  species  of  Vitis 

light  is  thrown  on  the  situation  by  the  consideration  of  the  seed- 
ling in  the  genus  Vitis.  Fig.  139  presents  the  stem  of  the  young 
individual.  There  are  seen  masses  of  wood  representing  leaf 
traces  still  in  position  in  the  woody  cylinder  of  the  stem.  The 
segments  under  consideration  are  clearly  bounded  on  either  side 
by  compound  rays,  while  in  the  wood  are  present  distinct  vestiges 
of  primitive  rays.  It  is  evident  that,  if  the  anatomy  of  the  seedling 
has  any  clear  bearing  on  the  problem  of  the  origin  of  the  type  of 
stem  presented  in  our  northern  species  of  grapevine,  primitive  rays 
were  once  present  in  the  intervals  of  wood  bounded  laterally  by 
the  large  or  compound  rays,  and  that  these  have  subsequently  been 


186  THE  ANATOMY  OF  WOODY  PLANTS 

lost.  The  disappearance  of  the  primitive  rays  from  the  bundles 
of  the  stem  is,  in  fact,  a  general  feature  of  organization  of  the  true 
vine  type  and  is  as  plainly  indicated,  for  example,  by  the  Ranun- 
culaceae  as  by  the  Vitaceae.  It  is  obvious  that,  as  a  result  of  the 
considerations  elucidated  above,  the  typical  vine  possesses  com- 
pound rays,  but  that  the  primitive  rays  have  usually  quite  dis- 
appeared. 


FIG.  139. — Diagram  of  the  organization  of  the  seedling  stem  of  a  northern  species 
of  Vitis. 

Very  frequently  the  vine  type  passes  almost  imperceptibly  into 
that  of  herbs,  as,  for  example,  in  the  genus  Clematis  among  the 
Ranunculaceae  and  the  genus  Aristolochia  belonging  to  an  order 
of  the  same  name.  It  is  convenient,  however,  to  take  up  the  ap- 
pearance of  the  herbaceous  type  in  an  exemplification  where  it  is 
not  linked  in  any  way  with  the  habit  or  conditions  of  organization 
found  hi  the  case  of  vines.  The  genus  Potentilla  will  serve  admir- 
ably in  the  present  connection.  To  reduce  the  illustrations  to  the 
smallest  possible  compass,  and  to  make  the  situation  at  the  same 
time  clearer,  a  diagrammatic  mode  of  representation  will  be 


THE  STEM 


187 


advantageous.  In  Fig.  1400  is  shown  the  stem  of  a  woody  Poten- 
tilla  (Potentilla  palustris).  There  are  five  broad  segments  of  the 
stem,  and  alternating  with  these  are  five  narrower  ones.  The  more 
exiguous  segments  are  related  to  the  leaf  traces  and  are  charac- 
terized, as  are  the  similar  regions  of  the  woody  axes  of  Casuarina 
and  Alnus  repre- 
sented in  earlier 
illustrations  in  this 
work,  by  the  con- 
spicuous absence 
of  the  vessels  pres- 
ent  in  the  remain- 
ing and  broader 
sectors.  Although 
in  the  outer  region 
the  sectors  under 
discussion  show  a 
complete  absence 
of  vascular  struc- 
tures, in  the  region 
near  the  pith  a  few 
elements  of  this 
nature  are  present. 
These  belong  to 
the  leaf  trace 
proper,  which  lies 
on  the  inner  sur- 
face of  the  narrower  segments  of  the  stem.  Not  only  is  the  leaf 
trace  segment  narrower  and  distinguished  by  the  absence  of  vascular 
structures,  except  in  the  actual  trace  itself,  but  it  is  depressed  below 
the  surface  of  the  adjoining  broader  sectors.  In  b  these  features 
are  indicated  on  a  larger  scale,  so  that  the  conditions  described 
become  the  more  obvious.  It  will  be  noted  under  the  more  favor- 
able conditions  of  greater  enlargement  that  the  cluster  of  vessels 
lying  on  the  inside  of  the  narrow  segment  and  near  the  pith  is 
flanked  on  either  hand  by  a  region  free  from  vessels.  In  c  is  shown 
the  organization  of  the  perennial  region  of  the  stem  in  another 


FIG.  140. — Diagrammatic  representation  of  stems  of 
species  of  Potentilla.    Explanation  in  the  text. 


1 88  THE  ANATOMY  OF  WOODY  PLANTS 

species  of  Potentilla,  namely,  P.  intermedia.  In  this  instance  the 
same  outstanding  and  depressed  segments  are  found  as  in  the 
previous  species,  but  the  conditions  otherwise  differ  in  important 
respects.  The  depressed  segments  corresponding  to  leaf  traces 
in  P.  intermedia  are  represented  in  solid  black,  which  is  the 
diagrammatic  expression  of  the  fact  that  they  are  no  longer  com- 
posed of  fibrous  elements,  as  is  the  case  with  the  species  discussed 
above.  The  substance  of  the  segments  is  now,  in  fact,  entirely 
parenchymatous,  and  they  are  comparable  to  the  compound  type 
of  ray  found  in  the  oak  and  similar  forms.  On  the  other  hand, 
the  fibrous  regions  of  the  cylinder  in  P.  palustris  supply  the  equiv- 
alent of  the  aggregate  rays  of  the  alder  and  of  types  which 
resemble  it.  In  d  is  shown  a  more  magnified  view  of  a  part  of  c. 
The  cylinder  presents  three  annual  rings,  which  are  seen  in  both 
outstanding  and  depressed  segments  of  the  stem.  In  the  inner- 
most annual  increment  of  the  depressed  foliar  region  is  imbedded 
the  leaf  trace,  marked  by  the  presence  of  vessels.  Obviously  it 
is  not  only  faced  by  parenchymatous  tissue  (indicated  in  black), 
but  is  likewise  flanked  by  the  same  substance  precisely  as  in  b  the 
trace  is  faced  and  flanked  by  the  fibrous  modification  of  the  wood. 
In  d  we  have  the  thick  cylinder  resulting  from  perennial  growth 
during  several  years.  If  the  external  region  of  this  cylinder  were 
cut  away  to  such  a  depth  as  to  reach  the  surface  of  the  leaf  trace, 
we  should  have  the  condition  which  is  found  in  an  annual  stem  of  the 
same  plant,  such  indeed  as  is  shown  in  e.  The  topographical  con- 
sequence of  the  thinning  out  of  the  woody  cylinder,  as  a  result  both 
of  less  massive  development  and  of  the  annual  habit,  is  the  increas- 
ing of  the  relative  importance  of  the  leaf  traces  as  components  of 
the  woody  cylinder.  This  means  that  the  storage  cells  which 
originally  not  only  flanked  but  also  subtended  the  traces  are  now 
confined  to  the  sides  or  flanks  of  the  foliar  traces  and,  in  fact, 
separate  these  from  the  adjacent  segments  of  the  cylinder.  This 
situation  is  distinctly  shown  in  e  for  the  cylinder  as  a  whole,  and  on 
an  enlarged  scale  for  a  single  foliar  trace  and  the  two  adjacent 
segments  of  the  woody  cylinder  in  /.  It  is  clear  from  the  diagram 
supplied  in  Fig.  140,  which  may  be  profitably  compared  with  that 
illustrating  the  topographical  and  evolutionary  features  of  the 


THE  STEM 


189 


rays  on  page  82,  that  the  herbaceous  stem  may  result  from  the 
breaking  up  of  the  originally  continuous  woody  cylinder  through 
the  formation  of  special  storage  devices  in  connection  with  traces 
of  the  leaves.  This  process  is  far  more  striking  in  the  more  delicate 
annual  stems  by  reason  of  the  slender  character  of  the  cylinder. 
It  is  apparent  that  the  conditions  found  in  the  stem  of  the  oak,  etc., 


A  B 

FIG.  141. — Diagram  of  stem  of  Labiatae.    Explanation  in  the  text 

as  regards  the  presence  of  large  foliar  rays,  elucidate  the  origin  of 
the  herbaceous  type. 

The  situation  illustrated  in  Fig.  140  is  not,  however,  by  any 
means  a  universal  expression  of  the  topography  in  herbaceous  stems. 
It  will  be  well  to  show  further  possibilities  in  this  direction  by 
reference  to  a  family  of  dicotyledons  much  higher  in  the  scale  than 
are  the  Rosaceae.  In  Fig.  1410  is  shown  the  perennial  stem  of 
the  common  garden  sage,  Salvia  officinalis.  The  outside  is  covered 
with  an  armor  of  periderm  within  which  lies  the  narrow  cortex, 


IQO  THE  ANATOMY  OF  WOODY  PLANTS 

in  turn  abutting  directly  on  the  fibrovascular  cylinder.  The 
latter  is  circular  in  outline  and  shows  no  variation  of  structure  in 
different  segments.  In  b  an  annual  branch  of  the  sage  is  diagram- 
matically  represented  so  as  to  indicate  the  essential  features  of 
organization.  The  stem  is  now  square  in  outline,  as  is  the  usual 
condition  in  herbaceous  representatives  of  the  Labiatae.  It  is  to 
the  flat  surfaces  of  the  quadrilateral  stem  that  the  paired  leaves 
are  attached.  An  examination  of  the  fibrovascular  cylinder  shows 
that  the  uniformity  of  structure  characteristic  of  the  round  peren- 
nial region  is  no  longer  present.  Opposite  the  flat  surfaces  the 
cylinder  is  much  thinned  out,  and  vessels  are  conspicuous  by  their 
absence,  except  in  small  clusters  marking  the  position  of  the  leaf 
trace,  which  will  pass  outward  at  a  higher  node.  Although  the 
fibrous  modification  of  the  structure  of  the  wood  in  relation  to  the 
foliar  fibrovascular  strands  is  found  here  as  in  the  case  of  Potentilla 
and  Alnus,  it  seems  clear  that  we  have  to  do  with  a  less  primitive 
phase  of  the  origin  of  the  herbaceous  type.  In  c  is  presented  a 
diagram  of  the  extremely  herbaceous  stem  of  Lamium  album. 
The  situation  as  regards  flattening  of  the  stem  in  connection  with 
the  quadrangular  structure  characteristic  of  the  axes  in  the  labiates 
is  virtually  the  same  as  in  the  annual  shoots  of  the  sage,  with  the 
difference  that  rays  are  no  longer  seen  in  the  purely  fibrous  and  very 
narrow  portions  of  the  cylinder  facing  the  flat  surfaces  and  flanking 
the  leaf  traces  on  either  side.  In  still  softer  and  more  herbaceous 
stems  the  fibers  in  turn  may  be  largely  or  entirely  replaced  by 
parenchyma,  so  that  the  cylinder  becomes  definitely  broken  up 
into  separate  strands.  Similar  conditions  in  a  general  way  present 
themselves  in  all  cases  in  which  the  axial  organs  of  dicotyledons 
become  annual  and  assume  the  herbaceous  texture — for  example, 
the  Solanaceae,  as  illustrated  by  the  woody  stem  of  the  bittersweet 
(Solanum  Dulcamara)  and  the  angular  soft  one  of  the  potato 
(S.  tuber osum)  and  of  the  tomato  (S.  esculentum).  In  the  two  last- 
named  species  the  furrows  of  the  stem  as  well  as  the  fibrous  or 
parenchymatous  regions  of  the  fibrovascular  cylinder  correspond 
in  position  to  the  leaves  which  are  a  factor  in  the  evolutionary 
processes  under  consideration.  The  Labiatae  and  Solanaceae 
have  been  chosen  to  illustrate  the  development  of  the  herbaceous 


THE  STEM  191 

type  in  higher  dicotyledonous  orders  because  the  simple  foliar 
trace,  which  is  a  feature  of  the  two  groups  under  discussion,  makes 
the  topographical  conditions  more  easily  understood.  The  Legu- 
minosae  in  the  case  of  the  garden  bean  or,  better,  of  the  garden  pea, 
illustrate  the  situation  appropriately  for  forms  with  plural  traces. 
At  the  same  time  the  two  types  just  mentioned,  as  well  as  the 
tomato  and  the  sage,  exemplify  the  principle  of  recapitulation, 
since,  in  all,  the  young  stem  is  round  and  woody  and  only  later 
assumes  the  angular  or  furrowed  configuration  of  maturity.  The 
hypocotyledonary  region  of  the  stem  in  the  bean,  for  example,  and 
the  lower  region  of  the  epicotyl  in  the  pea  both  present  a  circular 
and  complete  woody  cylinder.  The  developmental  evidence,  as 
also  that  derived  from  comparative  anatomy,  thus  clearly  points 
to  the  derivation  of  herbaceous  forms  from  woody  ones  and  not 
of  arboreal  perennials  from  annuals,  logically  following  from  the 
account  of  the  origin  of  the  structures  of  the  stem  in  dicotyledons 
originated  by  Sanio  and  disseminated  by  Sachs  and  De  Bary. 

Another  feature  of  organization  of  herbaceous  stems  which  often, 
although  not  invariably,  appears  is  the  degeneracy  of  the  rays  other 
than  those  broad  masses  of  storage  parenchyma  which,  as  has  been 
set  forth  in  the  foregoing  paragraphs,  flank  the  foliar  traces  in 
their  vertical  course  in  the  stem.  This  situation  is  well  illustrated 
among  the  ranunculaceous  representatives  of  the  Ranales,  for 
example,  the  buttercup,  the  meadow  rue,  and  the  clematis.  It 
has  already  been  discussed  in  sufficient  detail  in  connection  with 
the  vine  type  of  stem  in  a  former  paragraph  and  therefore  need  not 
be  referred  to  here. 

The  discussion  of  the  herbaceous  dicotyledons  brings  us  to  the 
consideration  of  the  stem  in  the  monocotyledons.  This  important 
group  of  plants,  which  in  physiological  efficiency  excels  all  the  other 
large  divisions  of  vascular  plants,  is  practically  entirely  herbaceous 
in  its  structure;  and  even  in  those  of  arboreal  habit  the  internal 
organization  is  that  of  herbs.  The  monocotyledons  on  account  of 
the  relative  simplicity  of  their  fibrovascular  strands  have  been 
referred  by  many  to  affinities  with  the  ferns  or,  on  the  basis  of  their 
habit,  to  relationship  with  the  older  gymnosperms.  These  attri- 
butions of  relationship  are,  however,  little  supported  by  either 


THE  ANATOMY  OF  WOODY  PLANTS 


anatomical  evidence  or  reliable  data  derived  from  the  study  of  the 
remains  of  plants  in  the  geological  strata. 

The  stem  in  this  large  and  important  group  is  distinguished  both 
by  the  structure  of  the  fibrovascular  strands  and  by  their  mode  of 
arrangement  in  a  transverse  section  of  the  stem  (Fig.  142) .  The  con- 
tinuous woody  cylinder  of  the  older  dicotyledons  in  the  herbaceous 
types  more  adapted  to  modern  climatic  conditions  in  temperate 

regions  has  already 
given  place  to  a  con- 
dition of  disconti- 
nuity.  In  the 
monocotyledons  the 
process  of  disinte- 
gration of  continuity 
has  gone  much  far- 
ther than  even  in  the 
herblike  dicotyle- 
dons. For  here  the 
separate  strands 
abandon,  under  the 
further  influence  of 
the  necessities  con- 
nected with  the  leaves,  the  original  circular  arrangement  for  a 
scattered  disposition  through  the  transverse  section  of  the  stem. 
The  high  efficiency  of  the  monocotyledons  in  the  elaboration  of 
foodstuffs  naturally  has  correlated  with  it  an  ample  provision 
of  numerous  fibrovascular  strands  for  conducting  the  assimilates 
into  the  stem  or  axis.  This  situation  brings  it  about  that  many 
traces  enter  the  stem  at  a  node  instead  of  the  three  or,  at  most, 
several  traces  which  pass  into  the  nodal  region  of  the  axis  in 
the  mass  of  dicotyledons.  This  multiplication  of  the  conducting 
strands  brings  with  it  complications  of  arrangement  in  the  stem, 
for  the  number  is  too  large  to  be  accommodated  on  the  periphery 
as  is  the  rule  even  among  herbaceous  dicotyledons.  As  a  con- 
sequence of  the  necessities  which  have  thus  arisen,  the  leaf  traces 
are  displaced  from  the  margin  of  the  central  cylinder  into  the  pith 
or  medulla.  It  is  thus  clear  that,  just  as  the  woody  stem  has  given 


FIG.  142. — Stem  of  Smilax 


THE  STEM  193 

place  to  the  herbaceous  one  under  the  influence  of  necessities  con- 
nected with  the  greater  elaborative  efficiency  of  the  leaves,  so  a 
still  further  accentuation  of  foliar  efficiency,  correlated  with  a 
corresponding  multiplication  of  foliar  traces,  has  led  to  the  further 
modification  of  the  cylinder  of  the  stem,  resulting  from  the  necessity 
of  accommodating  a  very  much  larger  number  of  foliar  traces  in  the 
transverse  area  of  the  cylinder.  This  is  usually  effected  by  moving 
the  foliar  traces  from  a  peripheral  position  to  the  medullary  region 
and  more  rarely  by  the  running  of  the  foliar  traces  for  one  or  more 
internodes  in  the  cortex.  The  multiplication  of  the  conductive 
strands  from  the  leaves  has  a  further  effect  in  the  organization  of 
the  bundles  which  will  be  discussed  in  a  later  paragraph. 

Not  only  in  the  arrangement  of  the  fibrovascular  bundles  do 
the  monocotyledons  differ  from  the  dicotyledons.  The  bundles 
themselves  differ  in  the  mass  of  monocotyledonous  forms  from  the 
great  majority  of  the  dicotyledonous  angiosperms  in  the  fact  that 
cambial  activity  is  absent.  Fig.  143  shows  the  general  topography 
of  the  bundle  in  Smilax  herbacea,  the  carrion  flower.  The  fibro- 
vascular strand  is  surrounded  by  a  well-marked  sclerenchymatous 
sheath  in  all  probability  corresponding  to  the  pericycle  in  lower 
forms.  It  is  only  rarely  that  in  the  group  under  discussion  endo- 
dermal  structures  are  well  developed  about  the  strands  in  the 
stem.  Generally  a  continuous  external  endodermal  sheath  sur- 
rounds the  bundles  in  common  and  no  internal  structures  of  this 
nature  can  be  made  out.  That  the  parenchymatous  tissue  lying 
between  the  bundles  in  the  stem  belongs  to  the  fundamental  system 
can  often  clearly  be  inferred  from  its  histological  character  where  its 
elements  strongly  resemble  those  of  the  cortex.  The  internal 
organization  of  the  bundle  is  distinguished  by  the  presence  of 
phloem  and  xylem  collaterally  disposed  in  relation  to  one  another. 
The  xylem  consists  for  the  most  part  of  vessels  and  parenchyma, 
and  the  vessels  are  of  the  porous  type.  The  elements  of  the  wood 
farthest  away  from  the  phloem  are  typical  spiral  and  ringed  proto- 
xylem.  No  cambium  or  zone  of  growth  ordinarily  is  interposed 
between  the  wood  and  the  phloem,  and  this  condition  has  gained 
for  the  fibrovascular  strands  of  the  monocotyledons  the  appellation 
of  closed  bundles.  It  has  often  been  supposed  that  the  closed 


194 


THE  ANATOMY  OF  WOODY  PLANTS 


character  of  the  bundles  in  the  group  is  an  indication  of  affinity 
with  the  pteridophytes,  but  this  view  seems  to  have  little  value 
when  it  is  realized  that  the  earlier  vascular  cryptogams  usually 


FIG.  143. — Bundle  of  Smilax  herbacea 

possessed  well-marked  secondary  growth.  Toward  the  periphery 
of  the  stem  in  each  fibrovascular  strand  lies  the  phloem,  composed 
mostly  of  sieve  tubes  and  their  companion  cells.  The  small 
elements  of  the  phloem  lying  on  the  very  outside  and  close  to  the 
bundle  sheath  represent  the  protophloem  and  are  often  in  a  some- 


THE  STEM  195 

what  swollen  condition.  The  sieve  tubes  in  the  monocotyledons 
are  of  a  high  type  and  are  characterized  by  terminal  walls  which  are 
generally  horizontal.  The  lateral  abortive  sieve  plates,  known  as 
lattices,  present  on  the  side  walls  of  the  sieve  tubes  in  woody 
dicotyledons  and  many  herbaceous  ones  as  well,  are  usually  absent 
in  the  group.  Both  phloem  and  xylem  consequently  represent 
a  high  condition  of  organization  and  indicate  an  advanced  phylo- 
genetic  position  for  the  monocotyle- 
dons. 

In  certain  monocotyledonous 
families  cambial  activity  has  been 
detected  in  some  instances.  This 
phenomenon  is  quite  generally  pres- 
ent in  the  grasses,  as  has  been  pointed 
out  by  Chrysler.  In  the  Liliaceae  and 
aroids  as  well  as  in  the  Cyperaceae  the 
presence  of  a  cambium  has  also  been 
recorded.  Fig.  144  shows  the  pres-  FIG.  144.— Bundle  of  Erianthus 

ence  of  secondary  growth  in  the  region     *"*>   showing  Presence   of 
,    .  .    .     _   .        .        „  cambial  activity  (after  Chrysler), 

of  the  node  in  Enanthus  Ravennae. 

Frequently  the  bundles  of  the  stem  in  monocotyledons  are  of 
the  closed  collateral  character,  but  in  some  cases,  particularly 
in  rootstocks  with  closely  approximated  nodes,  they  present  a 
different  type,  a  concentric  condition  which  differs  from  that 
found  in  the  Pteridophyta  by  the  fact  that  the  xylem  surrounds 
the  phloem  instead  of  the  phloem  forming  a  continuous  girdle 
about  the  xylem.  This  modification  of  the  bundle  is  known  as 
amphivasal  to  distinguish  it  from  the  amphicribral  condition  charac- 
teristic of  fern  allies.  A  strand  of  this  nature  is  shown  in  Fig.  145. 
The  phloem  occupies  the  center  of  the  figure  and  is  surrounded 
completely  by  the  tissues  of  the  xylem.  Bundles  of  this  type  are 
apparently  the  result  of  the  crowding  and  fusion  of  the  foliar 
strands  at  the  nodes,  for  it  is  in  the  nodal  region  that  they  are  best 
developed.  Where  the  nodes  are  remote  from  one  another,  the 
amphivasal  condition  may  entirely  disappear  in  the  internode  to 
reappear  at  the  next  node.  With  the  approximation  of  nodes, 
characteristic  of  many  creeping  monocotyledonous  rootstocks  with 


i96 


THE  ANATOMY  OF  WOODY  PLANTS 


tufted  leaves,  the  amphivasal  condition  becomes  continuous. 
Fibrovascular  strands  of  this  type  are,  in  fact,  a  common  feature 
of  organization  of  the  subterranean  regions  of  many  monocoty- 
ledons,' even  when  they  are  absent  in  the  stem.  This  situation  is 
well  exemplified  by  the  orchids  and  Iridaceae.  In  Gramineae  and 
Cyperaceae,  on  the  other  hand,  amphivasal  strands  are  present  in 
the  somewhat  remote  nodes  of  the  annual  flowering  stems,  but 

disappear  in  the  internodes, 
and  a  similar  situation  is  pres- 
ent in  the  perennial  subterra- 
nean stem  in  case  the  nodes 
are  not  crowded.  In  the  latter 
condition  the  amphivasal  state 
becomes  continuous.  In  the 
true  palms  (Principes)  and  in 
many  Scitamineae  the  amphi- 
vasal bundle  seems  to  have 
entirely  disappeared,  although 
this  is  a  matter  for  further 
investigation.  There  seems  to 
be  little  doubt  that  the  forma- 
tion of  amphivasal  bundles  in  the  region  of  the  nodes  is  an 
original  feature  of  organization  of  the  monocotyledons,  and  that 
it  persists  very  strongly  in  the  perennial  subterranean  stem  or 
rootstock,  but  may  entirely  disappear  in  the  annual  stem.  The 
amphivasal  condition  of  the  bundles  seems  to  be  definitely  cor- 
related with  the  numerous  leaf  traces  passing  into  the  stem  at  the 
node  in  the  monocotyledonous  angiosperms.  This  hypothesis  of 
its  origin  is  supported  by  the  fact  that  in  herbaceous  dicotyledons 
with  numerous  leaf  traces  the  occurrence  of  amphivasal  bundles 
at  the  node  can  frequently  be  observed.  The  Umbelliferae  may  be 
cited  as  an  excellent  example  of  this  condition. 

The  consideration  of  the  stem  has  now  been  completed,  and 
it  will  be  advantageous  in  conclusion  to  indicate  the  main  tendencies 
manifested  in  its  course  of  evolution  from  lower  to  higher  forms. 
It  has  been  noted  that  the  first  important  phase  in  the  evolution 
is  connected  with  the  primary  structures  of  the  fibrovascular  region. 


FIG.  145. — Amphivasal    concentric 
bundle  of  rootstock  of  Smilax. 


THE  STEM  197 

In  the  stems  of  lower  types  these  present  themselves  under  two 
general  conditions:  namely,  the  protostelic  and  the  siphonostelic. 
In  the  former  the  xylem  constitutes  a  solid  mass  without  any  central 
pith,  while  in  the  latter  the  fibrovascular  complex  is  thrown  into  a 
hollow  cylinder  in  which  gaps  are  frequently  present.  These  are 
generally  related  to  certain  of  the  appendages,  but  may  not  have 
any  topographical  connection  with  the  lateral  parts.  In  the 
siphonostelic  condition  internal  phloem  is  often  seen,  particularly 
in  the  lower  forms,  and  the  stele  or  central  cylinder  is  clearly 
separated  from  the  central  pith  or  medulla  by  the  presence  of  an 
internal  endodermis.  In  higher  types  the  internal  surface  of  the 
tubular  stele  tends  to  degenerate,  with  the  resulting  disappearance 
of  internal  phloem  and  endodermis.  The  original  state  of  the  wall 
of  the  hollow  cylindrical  stele  can  often  be  inferred  from  the 
character  of  the  foliar  strands  which,  in  accordance  with  a  general 
principle  to  be  enlarged  upon  in  a  later  chapter,  often  perpetuate 
a  situation  which  has  disappeared  in  the  stem.  Where  the  foliar 
traces  are  concentric  and  separated  from  the  fundamental  tissues 
by  a  well-marked  endodermis,  it  may  be  assumed  that  a  similar 
condition  was  formerly  present  in  the  organization  of  the  central 
cylinder  of  the  axis. 

A  further  interesting  phase  in  the  evolutionary  development 
of  the  vascular  cylinder  is  to  be  noted  in  connection  with  the 
progressive  degeneracy  of  the  primary  wood.  In  the  lower  forms 
this  region  of  the  xylem  is  well  developed,  and  both  by  its  massive 
character  and  by  the  lack  of  seriation  of  its  elements  it  is  clearly 
distinguishable  from  the  secondary  cylinder  which  surrounds  it. 
The  stem  of  higher  groups  presents  the  primary  structures  in  an 
obsolete  condition,  which  results  in  the  bringing  of  the  rays  of  the 
secondary  wood  into  apparent  contact  with  the  pith  or  medulla. 
This  advanced  state  of  modification  of  the  central  cylinder  is 
responsible  for  the  extremely  inappropriate  name  medullary  rays, 
which  is  applied  to  the  radial  parenchyma  of  the  secondary  wood. 
These  structures  are  best  called  wood  rays  or  simply  rays,  since 
primitively  in  the  stem  and  always  in  the  root  they  are  quite 
divorced  from  any  connection  with  the  pith. 


198  THE  ANATOMY  OF  WOODY  PLANTS 

The  next  important  modification  in  the  case  of  the  fibrovascular 
structures  of  the  stem  is  the  appearance  of  aggregations  of  rays  by 
reason  of  the  increased  efficiency  of  the  secondary  fibrovascular 
cylinder  in  regard  both  to  its  conducting  and  to  its  storage  elements. 
The  clusters  or  congeries  of  rays  originally  diffused  throughout 
the  woody  structure  tend  in  higher  groups  to  become  somewhat 
definitely  related  to  the  appendages.  By  further  progress  in  the 
evolution  of  radial  parenchyma  we  have  the  compound  and  diffuse 
types  of  rays  making  their  appearance,  the  former  particularly 
characteristic  of  herbaceous  types,  and  the  latter  of  the  mass  of 
dicotyledonous  forest  trees.  The  appearance  of  large  rays,  inter- 
rupting the  continuity  of  the  central  cylinder  and  bringing  about 
the  differentiation  of  separate  fibrovascular  strands,  is  a  particular 
feature  of  the  herbaceous  type  representative  of  later  geologic  time 
and  of  cooler  climatic  conditions. 

The  woody  cylinder  broken  up  into  fibrovascular  bundles  as 
the  result  of  the  evolution  of  storage  devices  in  connection  with  the 
appendages  in  the  herbaceous  dicotyledons,  in  the  monocotyledons 
undergoes  further  modification  in  relation  to  decrease  in  size  and 
increase  in  number  of  the  bundles  in  correlation  with  the  greater 
efficiency  and  consequently  more  numerous  conducting  strands 
of  the  leaves.  The  decrease  in  size  of  the  bundles  is  definitely 
related  to  the  loss  of  the  cambial  activity  which  characterized  the 
fibrovascular  structures  of  the  monocotyledonous  angiosperms  as 
a  whole.  The  increase  in  number  of  the  cauline  bundles,  resulting 
from  the  entry  of  very  numerous  leaf  traces  into  the  stem  at  the 
nodes,  brings  with  it  the  distribution  of  the  fibrovascular  strands 
throughout  the  transverse  section  of  the  stem.  The  monocotyle- 
dons may,  in  a  sense  not  altogether  figurative,  be  said  to  represent 
the  second  childhood  of  the  vascular  plants,  just  as  the  Pteridophyta 
constitute  its  true  infantile  phase  of  development.  It  is  necessary, 
however,  to  distinguish  very  clearly  between  the  primary  structures 
of  the  monocotyledonous  angiosperms  which  are  the  result  of  the 
loss  of  secondary  growth,  and  the  primary  structures  of  the  Paleo- 
zoic forms  which,  so  far  as  we  know,  are  a  primitive  feature  of 
organization.  • 


CHAPTER  XIV 


THE  LEAF 

A  general  definition  of  the  leaf  or  foliar  organ  has  been  supplied 
on  an  earlier  page.  The  leaf  is  important,  not  only  on  account 
of  its  features  of  structure  as  an  organ  of  vegetation,  but  also 
because  of  the  primitive  and  intimate  relation  between  it  and  the 
organs  of  reproduction.  The  latter  parts  will  be  discussed  in  two 
chapters  following 
the  present  one, 
while  the  main 
features  of  organi- 
zation of  the  leaf 
itself  will  occupy 
attention  in  the 
present  connec- 
tion. The  anat- 
omy of  the  foliar 
organs  of  the  cryp- 
togamic  forms, 
living  or  extinct, 
need  not  be  con- 
sidered at  this 
stage,  as  any  refer- 


FIG.  146. — Leaf  bundle  of  Cycas  revoluta.    Explana- 
tion in  the  text. 


ences  to  them 
which  are  neces- 
sary will  appear  in 
later  chapters  dealing  with  particular  groups. 

We  may  conveniently  begin  the  discussion  of  the  leaf  with  the 
most  primitive  type  of  living  gymnosperms,  the  Cycadales.  This 
group,  much  reduced  in  numbers  under  modern  conditions,  has  a 
type  of  organization  in  the  leaf  which  is  of  great  interest  from  the 
evolutionary  standpoint.  Fig.  146  presents  the  transverse  view  of 
one  of  the  bundles  of  the  main  axis  or  rachis  of  the  leaf  in  Cycas 

199 


200  THE  ANATOMY  OF  WOODY  PLANTS 

revoluta.  Toward  the  lower  side  of  the  figure  is  seen  the  phloem, 
composed  of  elements  which  in  the  main  are  arranged  in  regular 
radial  rows  indicating  an  origin  from  cambial  activity.  The  upper 
region  of  the  bundle  is  composed  of  xylem,  consisting  of  very  large 
elements  which  become  progressively  smaller  and  fewer  in  number 
in  the  downward  direction.  The  lower  point  of  this  aggregation  of 
cells  is  the  protoxylem.  This  region  is  surrounded  inferiorly  by 
a  few  rows  of  parenchyma  which  give  place  in  turn  to  more 


FIG.  147. — Transverse  section  of  leaf  of  Cycas  revoluta.    Explanation  in  the  text 

thick-walled  elements  of  the  xylem,  in  contact  with  the  regular 
radial  files  of  the  cambium.  The  lower  region  of  the  xylem  is  the 
so-called  centrifugal  xylem;  the  upper  portion  broadening  from  the 
narrow  protoxylem  is  the  centripetal  or  cryptogamic  xylem.  The 
first-mentioned  group  of  elements  of  the  xylem  are  pitted  in  their 
character,  as  will  be  made  clear  in  a  subsequent  longitudinal  illus- 
tration. The  latter,  the  so-called  cryptogamic  wood,  is  of  impor- 
tance from  the  evolutionary  standpoint  because  it  indicates  at  once 
a  clear  relationship  on  the  part  of  the  Cycadales  to  the  vascular 
cryptogams  and  to  the  lower  and  extinct  gymnosperms.  In  the 
next  figure  (Fig.  147)  is  represented  a  transverse  section  of  one  of  the 


THE  LEAF 


201 


divisions  of  the  leaf.  The  cryptogamic  wood  is  here  relatively  bet- 
ter differentiated  than  in  the  axis  of  the  leaf,  and  it  is  obvious  that 
the  seriation  of  its  elements  is  toward  the  upper  surface  of  the  foliar 
organ.  The  centrifugal  wood  is  very  slightly  developed.  In  the 
next  figure  (Fig.  148)  appears  a  foliar  bundle  of  Cycas  revoluta  in  its 
course  in  the  cortex  of  the  stem.  There  is  a  mass  of  irregularly 
arranged  primary  wood  in  the  center,  surrounded  by  the  radially 
disposed  second- 
ary wood,  which  in 
turn  is  followed  by 
the  tissues  of  the 
phloem.  The  phloem 
entirely  surrounds 
the  bundle,  and  this 
is  consequently  of 
the  concentric  type 
often  found  in  the 
living  ferns  and  their 
allies.  The  contrast 
in  organization  of 
the  bundles  of  the 
leaf  in  different 
parts  is  very  strik- 
ing. The  lower  con- 
centric region,  in 
accordance  with 

generally  accepted  principles  of  comparative  anatomy  to  be  eluci- 
dated later  in  a  special  chapter,  may  be  regarded  as  supplying 
evidence  of  the  former  concentric  character  of  the  woody  cylin- 
der of  the  stem  in  the  Cycadales,  a  hypothesis  entirely  justified 
by  the  existence  of  a  remarkable  group  of  fernlike  seed  plants 
in  the  Paleozoic,  known  as  the  Medulloseae,  from  which  on  both 
anatomical  and  reproductive  evidence  the  living  group  seems  to 
have  been  derived.  In  these  forms  the  bundles  of  the  stem  were, 
as  will  be  subsequently  shown,  often  concentric  in  their  organiza- 
tion. In  the  higher  region  of  the  leaf  trace  in  the  cycads  the 
concentric  character  is  lost,  but  the  centripetal  development  of 


FIG.  148. — Concentric  foliar  bundle  from  the  cortex 
of  Cycas  revoluta. 


202 


THE  ANATOMY  OF  WOODY  PLANTS 


part  of  the  wood  still  sufficiently  vouches  for  the  cryptogamic 
affinities  of  the  lowest  living  gymnosperms.  In  Fig.  149  is 
shown  the  longitudinal  view  of  the  leaf  trace  in  the  rachis  of  the 
leaf.  To  the  right  appears  the  phloem  made  up  of  elongated  ele- 
ments, the  sieve  tubes.  To  the  left  of  this  region  lie  certain  pitted 
tracheids  belonging  to  the  centrifugal  or  modern  wood.  Then  inter- 
vene a  few  parenchymatous  cells,  followed  by  the  spiral  elements  of 
the  centripetal  or  cryptogamic  wood.  These  pass  farther  to  the  left 


FIG.  149. — Longitudinal  section  of  a  foliar  bundle  in  Cycas  revoluta 

into  ringed,  scalariform,  and,  finally,  pitted  tracheids.  The  struc- 
ture of  the  leaf  in  the  Cycadales  is  clear  evidence  of  their  rela- 
tionship with  the  ancient  fernlike  gymnosperms,  which  in  turn  were 
doubtless  in  filiation  with  actual  ferns  in  the  remote  geologic  past. 
The  Cycadales  have  their  nearest  affinities  in  the  past  with  a 
group  of  gymnosperms,  the  Cycadofilicales  or  the  Pteridospermeae, 
which  were  so  strikingly  fernlike  in  their  habit  that  until  the  very 
end  of  the  last  century  they  were  regarded  as  ferns.  In  contrast 
to  the  Cycadales  the  mass  of  surviving  gymnosperms  have  taken 
their  origin  from  forms  which  are  known  as  Cordaitales.  These 
had  nothing  in  their  habit  which  recalls  the  ferns,  but  often 


THE  LEAF 


203 


present  a  strong  superficial  resemblance,  particularly  in  the  con- 
formation and  venation  of  their  leaves,  to  the  monocotyledons. 
.The  Cordaitales  had  leaves  varying  in  width,  but  always  char- 
acterized by  few  or  numerous  parallel  veins.  The  structure  of 
the  fibrovascular  bundles  constituting  these  veins,  however,  was 
entirely  different  from  the  features  of  organization  found  in  the 
nerves  of  the  leaves  of  living  monocotyledons.  Fig.  150  illus- 
trates the  general  organization  of  the  leaf  in  Cordaites  princi- 
palis.  The  veins  are  numerous  and  separated  from  one  another 
by  intervals  of  the  soft  mesophyll  of  the  leaf.  The  epidermis  is 


FIG.  150. — Transverse  section  of  the  leaf  of  Cordaites  principalis 

strengthened,  particularly  in  the  region  of  the  fibrovascular 
bundles,  by  hypodermal  bands  of  thick-walled  cells.  In  Fig.  151 
is  shown  an  enlarged  transverse  section  of  one  of  the  foliar  bundles 
of  the  species  under  consideration.  The  wood  was  entirely  cen- 
tripetal in  its  development  and  ended  on  either  flank  in  a  band  of 
thick-walled  elements  with  narrow  central  cavities.  The  zone  of 
thick-walled  elements  was  in  contact  externally  with  a  second  zone 
made  up  of  cells  with  thinner  walls  and  much  larger  lumina.  Both 
the  zones  just  described  inclose  the  phloem,  which  lies  below  the  mass 
of  centripetal  wood.  The  longitudinal  view  (Fig.  152)  throws  addi- 
tional light  on  the  organization  of  the  bundle.  The  double  sheath 
connected  with  the  flanks  of  the  xylem  and  forming  a  complete  circle 
about  the  phloem  is  composed  of  elongated  elements  with  bordered 
pits  on  their  walls.  These  are  known  as  transfusion  cells  and  are 


204 


THE  ANATOMY  OF  WOODY  PLANTS 


an  important  and  phylogenetically  interesting  component  of  the 
foliar  fibrovascular  strands  in  many  gymnosperms.  They  are  not 
a  salient,  and  certainly  not  a  primitive,  feature  of  organization  of 


FIG.  151. — Transverse  section  of  the  leaf  bundle  in  Cordaiies  principalis 

the  leaf  bundles  of  the  living  Cycadales  and  their  allies,  the  Paleozoic 
Medulloseae.  The  transfusion  sheath  of  the  foliar  bundles  in  the 
Cordaitales  did  not  always  manifest  the  complexity  that  it  shows 
in  the  case  of  C.  principalis  figured  above.  In  most  instances  only 
the  broader  and  shorter  thin-walled  elements  were  present,  the 


THE  LEAF 


205" 


elongated  and  thicker-walled  inner  sheath  being  absent.  The 
intimate  connection  between  the  centripetal  or  cryptogamic  wood 
and  the  transfusion  tissue  is  observed  in  all  cases.  It  is  clear 
accordingly  in  the  Cordaitales  that  the  transfusion  tissue  which,  as 
will  be  subsequently  shown,  plays  an  interesting  role  in  the  evolu- 
tionary history  of  higher  gymnospermous  groups  is  primarily  related 
to  the  centripetal  or  cryptogamic  wood. 


FIG.  152. — Longitudinal  section  of  a  leaf  bundle  in  Cordaites  principals 

The  organization  of  the  leaf  in  the  conifers  will  next  occupy  our 
attention,  since  the  present  chapter  deals  with  the  leaf  only  in  the 
features  which  are  of  general  evolutionary  importance.  Fig.  153 
shows  an  enlarged  view  of  the'whole  leaf  of  Pinus  strobus,  the  white 
pine.  The  epidermis,  reinforced  by  a  hypoderma,  surrounds  the 
median  green  part  of  the  leaf,  the  mesophyll,  in  which  lie  resin  canals 
and  a  single  fibrovascular  bundle.  The  latter,  exceptionally  for  the 
conifers,  is  marked  off  from  the  surrounding  fundamental  tissues 
by  a  well-defined  circular  endodermis.  The  fibrovascular  strand 
consists  of  xylem  entirely  centrifugal  and  mostly  secondary  in  its 
origin,  and  this  meets  with  the  phloem  on  its  lower  border.  Sur- 
rounding the  conducting  strand  of  the  leaf  is  a  mixture  of  ordinary 


206 


THE  ANATOMY  OF  WOODY  PLANTS 


parenchymatous  elements  containing  protoplasmic  structures  and 
short  empty  cells  with  bordered  pits.  These  are  the  transfusion 
elements,  and  it  is  easy  to  observe  that  they  are  most  abundant  on 
the  flanks  of  the  xylem  and,  in  fact,  take  their  origin  from  this 
general  region.  As  in  the  case  of  the  Cordaitales,  the  cells  of  the 
transfusion  tissue  surround  the  phloem  as  well  as  the  xylem.  An 
important  difference,  however,  is  the  absence  of  intermingled 


FIG..  153. — Transverse  section  o|  leaf  of  Pinus  Strobus 

parenchymatous  elements  in  the  girdle  of  transfusion  tissue  of  the 
older  group.  Transfusion  tissue  is  better  developed  in  the  leaf 
of  the  pine  than  in  any  other  of  the  subtribes  of  conifers.  In  most 
of  the  other  subgroups  of  the  Coniferales  the  transfusion  tissue  is 
mainly  confined  to  the  flanks  of  the  fibrovascular  bundle,  and  the 
conducting  strand  as  a  whole  is  not  sharply  separated  from  the 
remaining  tissues  of  the  leaf  on  account  of  the  absence  of  an  endo- 
dermis. 


THE  LEAF 


207 


A  link  between  the  conditions  found  in  the  case  of  Pinus  and 
the  anatomical  organization  of  the  Cordaitales  is  presented  by  a 
mesozoic  fossil  form  allied  to  Pinus  to  which  the  name  Prepinus  has 
been  given.  Fig.  154  illustrates  the  organization  oftheTeaf  in  this 
species.  It  is  bounded  by  flat  surfaces  as  a  result  of  contact  with 
surrounding  leaves  of  the  many-leaved  fascicle.  The  hypodermal 
structures  are  strongly  developed  as  in  the  Cordaitales  and  con- 
stitute the  same  strengthening  ribs  as  are  found  in  the  leaves  of 


FIG.  154. — Transverse  section  of  leaf  of  Prepinus  statenensis 

that  genus.  On  account  of  the  changes  resulting  from  fossiliza- 
tion  the  endodermis  is  not  so  distinct  as  in  living  pines,  and  the 
mesophyll  or  soft  substance  of  the  leaf  is  very  poorly  developed. 
The  bundle  is  surrounded  by  a  very  dense  transfusion  sheath  in 
which  all  the  cells  are  empty  and  provided  with  bordered  pits. 
Not  only  is  the  transfusion  sheath  massive  in  Prepinus  and  entirely 
composed  of  transfusion  tracheids  to  the  exclusion  of  paren- 
chymatous  elements,  but  it  is  further  complicated  by  the  presence 
of  an  internal  thick-walled  zone  comparable  with  the  similar 
structure  in  C.  principals  figured  above.  In  the  comparatively 


208 


THE  ANATOMY  OF  WOODY  PLANTS 


low  magnification  shown  in  the  photomicrograph  the  inner  zone 
of  the  transfusion  sheath  appears  merely  as  a  dark  boundary  sur- 
rounding the  fibrovascular  bundle  proper.  The  conducting  strand 
of  the  leaf  is  represented  by  the  xylem  alone,  the  phloem  having 
disappeared  during  fossihzation.  In  Fig.  155  the  central  region  of 
the  foregoing  is  represented  under  a  higher  degree  of  magnification. 


FIG.  155. — Portion  of  leaf  of  Prepinus  statenensis,  more  highly  magnified 

The  tracheary  character  of  the  transfusion  sheath  can  now  clearly 
be  discerned,  as  well  as  the  fact  that  it  contains  no  cells  of  the 
nature  of  parenchyma.  The  narrowness  of  the  cells  constituting  the 
inner  transfusion  sheath  is  also  now  quite  apparent.  In  Fig.  156 
are  shown  the  various  structures  of  the  fibrovascular  bundles  lon- 
gitudinally and  on  a  still  higher  scale  of  magnification.  The  outer 
transfusion  sheath  is  composed  of  elements  with  distinct  and 
rather  large  bordered  pits  which  abut  inwardly  on  the  narrow 
thick-walled  cells  of  the  inner  transfusion  sheath,  in  turn  connected 


THE  LEAF 


209 


with  the  xylem  of  the  fibrovascular  bundle.  Before  we  consider 
the  nature  of  this  relation  it  will  be  well  to  examine  more  carefully 
the  bundle  itself.  The  conductive  strand  is  represented  by  its 
xylem  only;  this  consists,  interestingly  enough,  of  two  regions — 
a  lower  one  in  contact  with  the  empty  space  once  occupied  by  the 
phloem,  and  an  upper,  consisting  of  distinct  rows  separated  by 
thinner-walled  elements.  The  latter  is  the  true  centripetal  or 


FIG.  156. — Longitudinal  section  of  the  leaf  of  Prepinus  statenensis.    Explanation 
in  the  text. 


cryptogamic  wood  which  in  Prepinus  alone  among  the  conifers  is 
present  in  a  typical  form.  The  lower  region  of  the  xylem  is  cen- 
trifugal and  corresponds  to  the  whole  of  the  xylem  in  the  leaf  of 
the  modern  Pinus'.  In  the  modern  or  centrifugal  region  of  the 
wood,  rays  are  distinctly  present,  and  these  pass  from  its  substance 
into  the  cavity  once  filled  by  the  phloem.  The  centrifugal  region 
of  the  wood  provided  with  rays  is  related  to  the  inner  transfusion 
sheath  on  its  flanks  alone.  This  relationship  corresponds  in  fact 
to  that  observed  in  the  case  of  living  pines,  as  is  shown  above  in 


2io  THE  ANATOMY  OF  WOODY  PLANTS 

Fig.  153.  The  most  significant  relation  of  the  xylem  in  the  leaf  trace 
is  by  means  of  the  series  of  centripetal  or  cryptogamic  elements. 
These  are  numerous  and  serve  to  bring  about  a  very  intimate  and 
copious  connection  between  the  bundle  proper  and  the  transfusion 
tissues.  There  is  good  reason  on  the  basis  of  the  anatomy  of 
Prepinus  and  the  Cordaitales  to  regard  the  transfusion  tissue  which 
characterizes  the  foliar  organization  of  all  but  the  very  lowest  of 

the  gymnosperms  as 
a  product  of  the 
,  differentiation  of  the 

A  centripetal  or  cryp- 

togamic wood.     In 
the  true  pines  of  the 
^^      Cretaceous  the 
centripetal  wood 
fc|    was  absent,  as  it  is 
^jjjjr     in  modern  species  of 
§y        the  genus,  but  the 
internal  transfusion 
sheath  was  often 
we^  developed,  thus 
showing  a  clear  filia- 
tion  with  Prepinus. 
FIG.  157.— Leafy  twig  of  Cosuorina  equisetifolia  (after   ^here   ftiQ   centrip- 
Solerderer).  F 

etal  xylem  has  dis- 
appeared, the  relation  between  the  bundle  and  the  transfusion  tissues 
occurs  on  the  flanks  of  the  centrifugal  wood.  It  will  be  obvious 
to  the  reader  that  the  tissues  surrounding  the  fibrovascular  bundle 
in  the  conifers  and  their  allies  are  of  considerable  interest  from 
the  evolutionary  standpoint.  The  transfusion  tissue  at  the  present 
time  has  a  significance  in  the  vegetative  leaves  of  all  but  the  lowest 
gymnosperms  in  connection  with  the  storage  of  water  and  the  con- 
ducting of  it  to  the  green  cells  of  the  mesophyll.  In  the  repro- 
ductive leaves  it  has  taken  on  another  but  equally  interesting 
function,  as  will  be  indicated  in  the  next  chapter. 

Transfusion  elements  are  found  in  the  leaves  of  all  seed  plants 
from  (and  excluding)  the  cycads  upward.     In  the  conifers  they  are 


THE  LEAF  211 

well  developed,  but  are  distinctly  degenerate  in  the  higher  subtribes 
of  the  group  and  are  less  well  developed  in  any  living  conifers  than 
they  are  in  Prepinus  and  Cretaceous  species  of  Pinus.  The 
Gnetales  often  show  the  transfusion  tissues  in  a  high  degree  of 
development.  The  small  leaves  of  Ephedra  naturally  show  them 
less  distinctly  than  the  large  persistent  foliar  organs  of  Welwitschia, 
where  they  constitute  a  very  conspicuous  feature  of  the  organiza- 
tion of  the  leaf.  In  Gnetum,  again,  in  accordance  with  its  higher 


FIG.  158. — Base  of  leaf  of  Casuarina  equisetifolia,  showing  transfusion  tissue 

systematic  position,  the  transfusion  sheath  is  less  conspicuous. 
Among  the  angiosperms  transfusion  tissues  are  present  in  the 
dicotyledons,  but  present  themselves  in  the  condition  typical  for 
the  higher  gymnosperms  only  in  the  genus  Casuarina.  Here,  as  is 
shown  in  Fig.  157  illustrating  the  organization  of  a  leafy  twig  of 
Casuarina  equisetifolia,  there  are  clusters  of  thick-walled  empty 
cells  flanking  the  leaf  traces.  This  relation  to  the  foliar  strands 
strikingly  resembles  that  found  in  the  higher  gymnosperms  and 
appears  to  be  good  evidence  of  the  primitive  position  of  this  interest- 
ing genus.  In  Fig.  158  is  shown  one  of  the  foliar  bundles  of  the 


212 


THE  ANATOMY  OF  WOODY  PLANTS 


genus  under  discussion  much  more  highly  magnified.  The  flanking 
relation  of  the  transfusion  elements  to  the  strand  of  xylem  is  now 
very  distinct  and  recalls  that  found  in  the  leaves  of  the  Cupres- 
sineae,  Taxineae,  etc.  Less  typical  manifestations  of  the  develop- 
ment of  transfusion  tissues  in  the  dicotyledons  are  provided  by 
those  forms  in  which  bundles  related  to  stomata  exuding  fluid 
water,  and  consequently  known  as  water-stomata,  terminate  below 


FIG.  159. — Transverse  section  of  leaf  of  Alnus  incana 

the  stomatic  pores  in  a  mass  of  wide,  short,  tracheary  elements. 
This  condition,  although  doubtless  derived  from  ancestral  gymno- 
spermous  structure,  has  departed  so  far  from  the  original  transfusion 
tissue  that  it  can  scarcely  be  included  in  the  same  morphological 
category. 

Transfusion  tissue,  as  will  be  apparent  from  the  last  paragraph, 
has  become  a  feature  of  very  subordinate  importance  in  the  organi- 
zation of  the  leaf  in  the  mass  of  the  dicotyledons.  The  general 
structure  of  the  foliar  organs  in  the  group  may  profitably  occupy 
attention  at  this  stage.  Fig.  159  reproduces  somewhat  diagram- 


THE  LEAF  213 

matically  the  anatomical  features  of  the  leaf  of  Alnus  incana. 
Above  is  a  layer  of  cells  containing  only  protoplasm  and  a  nucleus. 
A  similar  situation  is  presented  by  the  lower  surface  of  the  leaf, 
with  the  exception  that  the  continuity  of  the  epidermis  is  locally 
perforated  by  stomatic  openings.  The  elements  related  to  these, 
the  guard  cells,  are  distinguished  from  the  rest  of  the  epidermal 
layer  by  the  presence  of  chloroplastids.  The  upper  epidermis  is 
contrasted  with  the  lower,  not  only  by  the  absence  of  stomata,  but 
also  by  the  presence  of  a  rough  impervious  covering,  the  cuticle. 
Another  feature  of  interest  in  the  superior  epidermal  cells  is  the 
mucilaginous  modification  of  their  inner  walls,  and  this  is  expressed 
in  the  illustration  by  a  thick  layering.  The  central  portion  of  the 
leaf  is  occupied  by  the  mesophyll,  composed  of  the  palisade  (upper) 
region  and  the  spongy  (lower)  region.  The  development  of  palisade 
parenchyma  in  foliar  organs  is  definitely  rekted  to  the  amount  of 
insolation  or  exposure  to  light,  while  the  spongy  layer  is  less  well 
developed  when  the  leaf  is  strongly  illuminated,  and  becomes  much 
more  accentuated  in  foliar  organs  exposed  to  shade  and  a  damp 
atmosphere.  The  leaf  of  the  monocotyledons  supplies  little  in  a 
general  way  which  is  of  interest  from  the  standpoint  of  evolutionary 
anatomy,  except  the  occasional  persistence  of  cambial  activity. 


CHAPTER  XV 
THE  MICROSPORANGIUM 

The  microsporangium  of  the  vascular  plants  is  considered  first, 
because  there  can  be  no  question  that  it  is  the  primitive  type  of 
sporangial  structure  for  the  long  series  of  forms  which  are  character- 
ized by  the  possession  of  water-conducting  tracheary  tissues.  In 
the  lower  representatives  of  the  Vasculares  the  microsporangium 
is  the  only  type  present,  and  hi  the  heterosporous  cryptogams  and 
the  seed  plants  it  keeps  its  place,  with  little  change  of  its  original 
condition  of  organization,  sic]e  by  side  with  the  highly  modified 
megasporangium  and  seed.  The  relative  constancy  of  microspo- 
rangial  structures  makes  them  in  many  respects  of  the  greatest 
value  from  the  evolutionary  standpoint. 

If  the  liverworts  are  correctly  regarded  as  the  forms  nearest 
to  the  Pteridophyta  in  the  series  of  the  bryophytes,  there  can  be 
little  doubt  that  the  sporangium  in  its  primitive  form  of  sporogo- 
nium  is  the  forerunner  of  the  sporophyte  of  the  vascular  series. 
Professor  Bower  has  brought  forward  an  impressive  aggregation 
of  evidence  in  favor  of  the  hypothesis  that  the  sporophyte  is  the 
result  of  progressive  sterilization  of  sporogenous  tissue.  Although 
the  definite  mode  by  which  the  simple  sporogonium  of  the  thallose 
liverworts  gave  rise  to  the  sporophyte  of  the  Pteridophyta,  so 
complicated  in  its  internal  structure  and  external  organization,  is 
highly  speculative,  it  will  serve  a  useful  purpose  to  indicate  the 
main  probabilities  in  this  connection  based  for  the  most  part  on 
the  investigations  of  Leitgeb.  In  certain  liverworts,  such  as,  for 
example,  Corsinia  and  Boschia,  the  spore  sac  gives  rise  to  sterile 
cells  as  well  as  spores.  There  is  clear  evidence  in  these  and  in 
similar  cases  that  the  sterile  cells  are  modified  or,  as  Professor  Bower 
expresses  it,  sterilized  potential  sporogenous  cells.  In  many 
liverworts  the  sterile  cells  are  useful  in  distributing  the  spores. 
In  this  instance  they  are  much  elongated  and  have  their  walls 
spirally  thickened.  The  spirals  recoil  when  the  spores  are  ripe, 

214 


THE  MICROSPORANGIUM  215 

giving  them  an  impetus  which  scatters  them  over  the  surface  of 
the  ground.  These  spore-distributing  mechanical  cells  are  known 
as  elaters.  In  certain  liverworts,  such  as  Pellia  and  Aneura, 
the  elaters,  in  addition  to  occurring  loosely  among  the  spores 
which  they  serve  to  scatter,  are  aggregated  in  a  compact  elon- 
gated mass  at  one  end  or  the  other  of  the  spore  sac  or  theca. 
This  longitudinal  cluster  of  elaters  may  be  regarded  with  some 
degree  of  probability  as  the  prototype  of  the  fibrovascular  bundle 
of  the  Pteridophyta  and  higher  groups  of  vascular  plants.  In 
Anthoceros  and  allied  forms  the  cluster  of  elaters,  known  as  the 
columella,  becomes  a  much  more  important  structure  and  traverses 
the  sporangium  from  end  to  end.  Laterally  at  intervals  it  gives 
off  transverse  ramifications  which  divide  the  mass  of  spores  into 
separate  clusters,  and  these  may  perhaps  be  regarded  as  the  proto- 
types of  the  sporangia  found  in  the  vascular  series.  The  situation 
in  the  horned  liverwort  Anthoceros,  in  which  there  is  an  extensive 
columella  with  lateral  ramifications,  gives  some  support  for  the 
hypothesis  of  the  derivation  of  the  sporophyte  from  the  sporogonium 
by  the  sterilization  of  potential  sporogenous  tissues.  The  mode 
in  which  the  organs,  leaf,  stem,  and  root  arose  from  such  a  primitive 
condition  of  organization  is  much  disputed,  since  none  of  the  hypo- 
thetical transitional  forms  between  the  moss  capsule  and  the  sporo- 
phyte of  the  higher  plants  have  yet  been  discovered.  There  can  be 
little  doubt,  however,  in  a  general  way  that  the  sporogonium  is  the 
forerunner  of  the  sporophyte  and  that  the  elater  is  the  prototype 
of  the  tracheid  in  vascular  plants. 

It  will  be  clear  from  the  foregoing  statement  that  the  sporangium 
is  so  intimately  involved  in  the  primitive  organization  of  the  most 
ancient  spore-producing  members  that  it  is  entirely  proper  to 
consider  it  a  definite  organ  of  higher  plants  on  a  footing  of  equality 
with  the  root,  stem,  and  leaf.  If  this  view  of  the  matter  is  sound, 
obviously  no  very  useful  purpose  can  be  served  by  the  examination 
of  the  development  of  particular  cells  of  the  foliar  organs  which  are 
in  some  way  or  other  related  to  the  origin  of  sporangia.  It  is 
likely,  moreover,  that  the  sporangia  of  the  Pteridophyta  give  us  on 
the  whole  a  less  accurate  picture  of  the  original  type  of  sporangium 
than  those  of  the  lowest  seed  plants.  It  is  therefore  appropriate 


2l6 


THE  ANATOMY  OF  WOODY  PLANTS 


in  the  present  connection  to  begin  the  discussion  of  the  sporangium 
with  the  consideration  of  the  situation  presented  by  the  lowest 
living  gymnosperms,  the  Cycadales.  Fig.  160  shows  the  organiza- 
tion of  a  microsporangium  in  Zamia  muricata.  The  structure  in 
question  is  covered  on  the  outside  by  an  envelope  of  thick-walled 
cells  which  in  the  condition  of  maturity  determine  its  dehiscence. 
The  mechanical  structure  is  known  as  the  annulus  and  is  of  great 

importance  in  bringing  about  the 
distribution  of  the  spores,  particu- 
larly in  the  lower  Vasculares,  in 
which  it  takes  the  place  of  the 
elaters  found  in  many  of  the  liver- 
worts. The  annulus  is  plainly  an 
epidermal  structure,  both  because 
it  is  actually  the  external  layer  of 
the  sporangium  and  because  its  con- 
tinuity is  interrupted  by  the  pres- 
ence of  stomata.  These  can  be  seen 
in  the  figure  in  profile  view. 

The  situation  in  regard  to  the 
annulus  in  the  Pteridophyta  may  be 
briefly  summarized.  In  lower  forms 
the  thick-walled  epidermal  cells 
which  serve  as  the  mechanism  for 
the  opening  of  the  sporangium  are 
massive  in  their  development,  while  in  the  higher  forms  of  the 
vascular  cryptogams  the  amount  of  mechanical  tissue  tends  to 
become  more  and  more  restricted.  Fig.  i6ia  shows  the  structure 
of  the  sporangium  and  its  annulus  in  Sdaginella.  The  mechanical 
layer  in  this  case  is  extensive  and  is  almost  coextensive  with  the 
surface  of  the  spore  sac.  In  contrast  to  the  conditions  shown  hi 
Selaginella  are  those  presented  by  many  of  the  ferns.  In  Fig.  i6i& 
is  reproduced  the  organization  of  the  sporangium  of  Polypodium 
vulgare  as  an  illustration  of  the  higher  type  of  annulus  in  the 
Pteridophyta.  The  opening  mechanism  here  constitutes  an  incom- 
plete vertical  ring  and  in  consequence  literally  merits  the  name  of 
annulus.  The  further  consideration  of  the  types  of  annulus 


FIG.  160. — Sporangium  of 
Zamia  muricata. 


THE  MICROSPORANGIUM  217 

presented  by  those  forms  included  under  the  general  heading  of 
Pteridophyta  need  not  occupy  our  attention  in  the  present  con- 
nection, important  as  these  structures  are  from  the  standpoint  of 
taxonomy  and  the  evolution  of  particular  groups. 

Returning  to  the  seed  plants,  we  find  that  the  Cycadales  are 
the  only  living  forms  in  which  the  organization  of  the  mechanical 
tissues  of  the  sporangium  corresponds  with  that  generally  existing 


CL 

FIG.  161. — Sporangium  of  Sdaginella  spc.  and  of  Poly  podium  vulgar  e 

in  the  Pteridophyta;  and  in  accordance  with  this  general  situation 
it  will  be  made  clear  that  the  epidermis  has  not  an  important  rela- 
tion to  the  distribution  of  the  microspores  of  plants  producing 
seeds.  The  interesting  genus  Ginkgo  will  serve  to  illustrate  advan- 
tageously the  situation  for  the  lower  living  seed  plants.  Fig.  162 
shows  the  organization  of  one  of  the  two  sporangia  of  the  micro- 
sporophyll  of  this  genus  as  viewed  in  longitudinal  vertical  section. 
Clearly  the  cells  of  the  epidermis  are  thin-walled  and  can  perform 
no  important  office  in  the  openings  of  the  spore  cavities.  Beneath 
the  epidermis  is  found  a  broad  zone  of  cells  provided  with  barred 


218 


THE  ANATOMY  OF  WOODY  PLANTS 


thickenings  in  their  walls,  resembling,  in  fact,  short  tracheids  with 
reticulate  thickenings.  By  following  the  mass  of  reticulate  mechan- 
ical cells  to  the  proximal  end  of  the  sporangium  we  find  that  they 
are  continuous  with,  and  pass  by  imperceptible  transitions  into, 
transfusion  elements  related  to  the  fibrovascular  bundles  of  the 
sporophyll.  In  Ginkgo  it  is  evident  that  the  opening  mechanism 
of  the  sporangium  is  a  derivative  of  the  fibrovascular  system,  and 

does  not  take  its  origin  from 
the  cells  of  the  epidermis,  as 
is  the  case  with  the  annulus  of 
the  Cycadales,  and  forms  lower 
in  the  scale  of  vascular  plants. 
The  situation  is  so  important 
in  this  respect  that  it  is  worthy 
of  being  given  a  special  nomen- 
clature.  In  those  types  in 
which  the  dehiscence  depends 
on  epidermal  mechanisms,  in- 
cluding the  Pteridophyta  and 
the  very  lowest  seed  plants, 
the  term  ectokinetic  may  be  ap- 
plied to  the  sporangium.  On 
the  other  hand,  in  the  long 
series  of  forms  beginning  with 
Ginkgo  and  ending  with  the 
monocotyledons,  in  which  the  opening  mechanism  is  of  internal 
origin  and  related  to  the  fibrovascular  system,  the  designa- 
tion endokinetic  definitely  indicates  the  origin  of  the  apparatus 
involved. 

It  is  necessary  to  examine  more  in  detail  the  conditions  found 
in  the  walls  of  the  sporangia  of  the  forms  above  Ginkgo.  In  Fig.  163 
is  represented  a  longitudinal  vertical  section  of  the  microsporan- 
gium  in  Pseudolarix  Kaempferi,  a  representative  of  the  Abietineae 
or  pinelike  conifers.  Here  the  situation  resembles  in  a  general  way 
that  found  in  the  case  of  Ginkgo,  for  the  opening  of  the  sporan- 
gial  sac  is  due  to  the  presence  of  reticulately  thickened  cells  which 
are  likewise  related  to  the  fibrovascular  system  of  the  trace  of  the 


FIG.  162. — Sporangium  of  Ginkgo  biloba 


THE  MICROSPORANGIUM 


219 


microsporophyll.  As  in  Ginkgo,  the  transition  from  the  mechan- 
ical tissue  to  the  tracheids  of  the  fibrovascular  bundle  is  effected 
by  transfusion  elements.  An  interesting  difference  between  the 
situation  found  in  Ginkgo  and  that  illustrated  in  the  Abietineae  and 
other  conifers  is  the  fact  that  the  mechanical  elements  invade  the 
epidermis  in  Pseudolarix,  but  fail  to  do  so  in  the  more  primitive 
genus.  In  other  members  of  the  Abietineae,  particularly  where 
the  wall  of  the  sporangium  is  very  thin,  the  mechanical  tissue 
becomes  correspond- 
ingly  reduced  in 
amount  and  no 
longer  shows  any 
clear  relation  to  the 
fibrovascular  sys- 
tem. In  the  sub- 
tribes  of  Coniferales 
above  the  Abie- 
tineae this  condition, 
in  fact,  becomes  the 
rule,  and  so  abortive 
does  the  mechanical 
layer  become  that  it  is  represented  by  the  reticulately  thickened 
cells  of  the  epidermis  alone.  It  thus  results  that  the  opening  of  the 
sporangial  cavity  is  once  more  effected  by  superficial  cells,  but  the 
situation  here  represented  should  be  carefully  distinguished  from 
that  in  the  Pteridophyta  and  Cycadales,  since  it  is  the  result  of  the 
invasion  of  the  epidermis  by  mechanical  tissues  of  fibrovascular 
origin.  Subsequently,  when  the  dehiscing  mechanism  was  reduced, 
the  epidermis  once  more  became  secondarily  the  seat  of  the  opening 
device.  It  is  not  without  significance  in  this  connection  that  the 
araucarian  conifers,  which  are  often  regarded  as  the  lowest,  present 
the  sporangial  arrangements  of  the  mass  of  conifers  in  which  the 
dehiscing  mechanism  is  reduced  to  its  lowest  terms,  and  not  that 
luxuriant  and  apparently  primitive  condition  exemplified  by  Ginkgo 
and  the  Abietineae. 

In  the  Gnetales  the  small  sporangia  are  not  characterized  by  the 
presence  of  a  very  well-developed  opening  device.     The  situation 


FIG.  163. — Sporangium  of  Pseudolarix  Kaempferi 


220  THE  ANATOMY  OF  WOODY  PLANTS 

in  the  case  of  the  angiosperms,  however,  is  different.  In  Fig.  164, 
showing  transverse  sections  of  the  anthers  of  a  tulip  and  a  honey- 
suckle, the  mechanical  tissues  are  clearly  differentiated  and  occupy 
an  entirely  internal  position  precisely  as  in  Ginkgo.  There  is  a 
very  important  difference  between  the  situation  presented  by 
the  angiosperms,  whether  dicotyledons  or  monocotyledons,  and 
Ginkgo  and  the  Abietineae.  In  the  higher  group  the  mechan- 
ical tissues,  constituting  the  so-called  fiber  layer  of  the  anther 


FIG.  164. — Sporangia  of  tulip  and  honeysuckle 

wall,  have  no  longer  any  relation  to  the  fibrovascular  bundles  of 
the  filament.  In  the  case  of  the  angiosperms  the  relation  once 
existing  between  the  fibrovascular  system  and  the  opening  mechan- 
ism has  apparently  been  lost.  The  dehiscing  apparatus  is,  however, 
still  in  a  good  state  of  development  and  in  this  respect  contrasts  to 
the  situation  presented  by  the  Gnetales  and  the  higher  Coniferales. 
The  structure  of  the  walls  of  the  microsporangia  of  the  vascular 
plants  from  Ginkgo  upward  is  highly  interesting  from  the  stand- 
point of  the  doctrine  of  descent.  In  the  lower  members  of  this 
series  the  opening  device  of  the  sporangium  is  clearly  in  relation  to 
the  transfusion  tissue  connected  with  the  fibrovascular  bundles  of 
the  reproductive  leaves.  In  the  Cycadales,  the  lowest  living  seed 


THE  MICROSPORANGIUM  221 

plants,  we  find  a  complete  absence  of  typical  transfusion  tissue  in 
the  leaves,  although  its  presence  has  been  erroneously  described 
for  the  group.  The  cycadean  gymnosperms  in  the  absence  of 
foliar  transfusion  tissue  resemble  the  true  ferns,  which  are  also 
characterized  by  the  exclusion  of  tracheary  tissues  belonging  to 
this  category  from  their  foliar  organs,  whether  vegetative  or 
reproductive.  It  is  highly  significant  that  an  epidermal  sporangial 
mechanism  and  the  absence  of  transfusion  tissue  are  features  which 
alike  mark  the  Pteridophyta  and  the  seed  plants  most  nearly  allied 
to  these.  Beginning  with  the  Ginkgoales  and  proceeding  upward, 
we  find  transfusion  tissue  progressively  taking  the  place  of  the 
centripetal  or  cryptogamic  wood  in  the  vegetative  leaves;  and  in 
the  reproductive  leaves  the  transfusion  tissues  or  structures 
definitely  associated  with  them  assume  the  function  of  opening  the 
sporangium  at  the  time  of  the  ripening  of  the  spores.  The  cor- 
rectness of  this  interpretation  of  the  situation  is  best  seen  in 
Ginkgo,  in  which  in  the  lower  region  of  the  sporophyll  the  trans- 
fusion tissues  are  developed  very  much  after  the  manner  in  which 
they  present  themselves  in  the  case  of  the  vegetative  leaves. 
In  the  upper  region  of  the  sporophylls,  bearing  the  microsporangia, 
the  transfusion  elements  grade  imperceptibly  into  the  reticulately 
thickened  mechanical  tissues  of  the  sporangial  walls.  In  the  sub- 
tribe  of  the  conifers  which  is  beginning  to  assume  importance  as  a 
candidate  for  the  most  primitive  phylogenetic  position  in  the  group 
(namely,  the  Abietineae) ,  we  find  the  transfusion  zone  not  only  well 
developed  in  the  vegetative  leaves  of  both  living  and  fossil  repre- 
sentatives, but  likewise  occurring  under  highly  significant  condi- 
tions in  relation  to  the  sporangial  mechanisms.  In  the  remaining 
gymnosperms  the  mechanical  tissue  shows  a  strong  tendency  to 
become  reduced  in  amount  and  loses  all  direct  relationship  to  the 
fibrovascular  tissues  proper.  In  the  angiosperms,  as  has  been 
pointed  out  above,  the  fiber  layer  characteristically  present  in  the 
anther  wall  is  well  developed,  but  no  longer  has  any  relation  what- 
soever to  the  fibrovascular  system. 

In  conclusion,  it  may  be  stated  that  the  opening  mechanism  of 
the  sporangia  of  the  Pteridophyta  and  of  the  lowest  gymnosperms 
is  epidermal  in  its  origin,  while  that  of  the  seed  plants  from  Ginkgo 


222  THE  ANATOMY  OF  WOODY  PLANTS 

upward  is  clearly  derived  from  transfusion  tissue.  This  category 
of  tissue  is  the  final  stage  of  persistence  of  the  protean  centripetal 
or  cryptogamic  wood  of  the  lowest  vascular  plants.  In  the  angio- 
sperms  the  mechanical  tissues  in  the  walls  of  the  anthers  exemplify 
the  highest  level  of  survival  of  the  old  centripetal  wood  of  the 
Pteridophyta  and  the  lowest  gymnosperms.  In  fact,  the  so- 
called  fiber  layer  of  the  anther  in  the  case  of  the  angiosperms 
supplies  the  clearest  instance  of  the  persistence  of  this  ancestral 
structure  outside  of  that  most  conservative  of  all  organs,  the  root, 
in  which,  as  has  been  made  clear  in  an  earlier  chapter,  it  still 
maintains  its  pristine  development  in  the  primary  organization. 
The  two  mechanisms  correlated  with  sporangial  dehiscence  pre- 
viously described  may  appropriately  be  designated  as  ectokinetic 
and  endokinetic. 


CHAPTER  XVI 
THE  MEGASPORANGIUM  AND  SEED 

In  the  Pteridophyta  the  phenomenon  of  heterospory  has  de- 
veloped in  many  different  groups.  The  result  of  the  realization 
of  this  condition  has  been  the  appearance  of  smaller  sporangia 
producing  numerous  small  spores  known  as  microspores  and  of 
larger  ones  giving  rise  to  a  few  krge  spores  designated  as  mega- 
spores  or  macrospores.  In  the  case  of  the  sporangia  which  give 
rise  to  megaspores,  or  the  megasporangia,  the  conditions  connected 
with  opening  are  the  same  as  those  exhibited  by  the  mass  of  Pteri- 
dophyta; in  other  words,  the  spore  sacs  are  ectokinetic  and  owe 
their  dehiscence  to  the  activity  of  a  mechanical  layer  derived  from 
the  epidermis.  As  this  situation  has  been  sufficiently  discussed 
in  the  previous  chapter,  it  will  not  be  profitable  to  return  to  the 
matter  here.  Megasporangia  in  the  proper  sense  of  the  word, 
wherever  they  occur,  are  ectokinetic. 

It  has  been  recognized  since  the  times  of  the  great  German 
morphologist  Hofmeister  that  seeds  represent  modified  mega- 
sporangia. This  view  of  the  origin  of  seeds  is  justified,  not  only 
by  their  anatomical  structure  and  by  the  cytological  conditions 
observed  in  the  development  of  the  endosperm,  but  also  by  the 
actual  persistence  of  the  megaspore  membrane  in  the  seeds  of 
many  of  the  lower  gymnosperms.  If  any  remaining  doubt  existed 
as  to  the  origin  of  seeds  from  megasporangia,  it  would  be  removed 
by  the  discovery  of  certain  interesting  structures  in  the  case  of 
Paleozoic  lycopods  which  present  at  the  same  time  many  of  the 
distinctive  characteristics  of  megasporangia  and  seeds.  So  strik- 
ingly does  one  of  these  (described  by  Scott)  resemble  a  seed  in  its 
external  appearance  that  it  was  for  a  time  actually  regarded  as  one, 
until  its  internal  organization  revealed  its  anomalous  character. 
Fig.  165  illustrates  the  vertical  section  of  Lepidocarpon  Lomaxi, 
the  seedlike  fructification  of  a  lepidodendrid.  Internally  is  shown 
a  mass  of  cells,  the  gametophyte,  surrounded  by  a  heavy  dark  line, 

223 


224 


THE  ANATOMY  OF  WOODY  PLANTS 


the  section  of  the  megaspore  membrane.  In  addition  to  the 
germinated  megaspore  shown  in  the  figure,  three  other  abortive 
megaspores  make  their  appearance  at  an  earlier  stage,  and  these 
have  thicker  walls  than  the  functional  spore.  These  are  not  shown 
in  the  late  stage  of  development  appearing  in  the  figure.  Not  only 
does  the  megaspore  retain  its  thick  membrane  in  the  fructification 
of  Lepidocarpon,  but,  contrary  to  the  conditions  found  in  typical 

seeds,  the  mechanical  layer  of 
the  megasporangium  likewise  is 
well  developed  and  was  prob- 
ably capable  of  dehiscence. 
The  integument  with  which 
true  seeds  are  provided  is  rep- 
resented in  this  foreshadowing 
of  seminal  structure  by  the 
upfolded  edges  of  the  sporo- 
phyll.  The  absence  of  an 
apparatus  for  receiving  the 
microspores  or  pollen  likewise 
differentiates  the  fructification 
under  consideration  from  the 
seeds  of  even  the  lowest  of  the 
seed  plants. 

In  Fig.  1 66  is  shown  the 
longitudinal  view  of  another 
seedlike  structure  from  the  Carboniferous  of  England  known  as 
Miadesmia.  Here  the  resemblance  to  the  real  seed  is  much  more 
marked  than  in  Lepidocarpon.  The  sporophyll  so  completely 
involves  the  megasporangium  that  only  a  small  aperture  is  left 
which  corresponds  physiologically,  although  not  morphologically, 
to  the  micropyle  of  the  seed  in  the  true  seed  plants.  Within  the 
"integument"  is  inclosed,  not  only  the  megasporangium,  but  also 
the  ligule.  The  megasporangium  is  much  less  typical  than  that  of 
Lepidocarpon,  for  its  mechanical  layer  fails  to  develop  and  it  pro- 
duces only  one  spore  in  contrast  to  the  four  that  come  into 
existence  in  the  case  of  the  Lepidocarpon,  There  is  no  good  reason, 
however,  to  regard  the  structure  here  described  as  representing  a 


FIG.  165. — Seedlike  sporangium  of 
Lepidocarpon  (after  Scott). 


THE  MEGASPORANGIUM  AND  SEED  225 

true  seed  any  more  than  that  delineated  in  connection  with  the 
last  paragraph. 

An  interesting  condition  has  been  described  by  Miss  Lyon 
in  the  American  species  of  Selaginella,  S.  apus,  and  S.  rupestris. 
Here  the  spores  germinate  and  are  fertilized  within  the  mega- 
sporangia,  a  condition  favored  by  the  fact  that  the  micro- 
sporangia  are  situated  in  the  upper  region  of  the  cones.  The 
microspores  undergo  development  unshed,  and  on  a  wet  day  the 
antherozoids  to  which  they 
give  rise  are  able  to  make 
their  way  to  the  lower 
region  of  the  strobilus, 
where  the  germinated 
megaspores  present  their 
archegonia  for  fertilization. 
The  development  of  the 

embryo    takes    place  after          FIG.  1 66 — Seedlike  sporangium  of  M iadesmia 

the  union  of  the  sexual  ele-  (from  Seward'  af ter  Scott)' 
ments,  and  the  sporelings  later  grow  out  among  the  leaves  of  the 
cone.  The  situation  represented  by  the  two  species  mentioned 
appears  to  be  somewhat  general  for  the  genus  and  throws  an 
interesting  light  on  the  conditions  which  were  probably  present  in 
Lepidocarpon  and  Miadesmia. 

The  most  ancient  types  of  seeds  known  to  us  have  an  organiza- 
tion differing  in  important  particulars  from  the  seedlike  structures 
described  in  the  two  preceding  paragraphs.  In  the  first  place,  the 
most  antique  seeds  are  provided  with  a  true  integument  and  are 
not  merely  wrapped  in  the  sporophyll  as  a  whole.  Secondly,  they 
present  a  very  important  feature  in  the  presence  of  a  so-called 
"pollen  chamber"  which  receives  the  microspores  and  provides  a 
fluid  in  which  they  may  undergo  germination  and  later  effect 
fertilization. 

A  primitive  type  of  seed  is  diagrammatically  represented  in 
Fig.  167.  The  megasporangium  appears  within  the  integument 
which  covers  it  almost  completely,  so  that  communication  with  the 
outer  world  is  only  by  a  narrow  canal  at  the  apex  known  as  the 
micropyle.  The  megasporangium  is  without  any  mechanical  layer 


226 


THE  ANATOMY  OF  WOODY  PLANTS 


such  as  appears  in  the  seedlike  structures  described  above  for  the 
lycopods.  Such  a  kyer  was  doubtless  originally  present,  but  has 
ceased  to  be  necessary  as  a  protection  on  account  of  the  shelter 
afforded  by  the  integument ;  moreover,  it  could  no  longer  be  func- 
tionally useful  in 
opening  the  spo- 
rangium since  the 
megaspores  in  the 
case  of  true  seeds 
are  permanently 
inclosed.  The 
elimination  of  the 
ectokinetic  me- 
chanical layer  of 
the  older  seeds 
must  lie  far  in  the 
geological  past,  be- 
cause no  evidence 
of  its  presence  has 
been  observed  in 
the  oldest  seeds 
with  the  structure 
of  which  we  are 
acquainted.  The 
pointed  apex  of  the 
megasporangium  is 
occupied  by  a 
cavity,  the  pollen 
chamber,  in  which 
the  pollen  grains  or  microspores  come  to  rest  before  germination. 
This  cavity  has  its  capacity  much  reduced  by  the  presence  of  a 
central  column  known  as  the  columella.  Below  the  pollen  cham- 
ber lies  the  germinated  megaspore  with  its  somewhat  thickened 
megaspore  membrane.  The  membrane  incloses  the  gametophyte, 
bearing  the  archegonia  in  its  upper  region.  Surrounding  the  part 
of  the  seed  corresponding  to  the  megasporangium  and  fused  with 
it,  except  in  the  upper  region,  is  the  integument,  and  this  consists 


FIG.  167. — Diagram  of  seed  of  a  cycad 


THE  MEGASPORANGIUM  AND  SEED 


227 


of  a  hard  inner  layer  known  as  the  sclerotesta  and  a  softer  outer 
one  which  contains  mucilage  canals  and  to  which  the  name  of 
sarcotesta  is  applied.  Both  sclerotesta  and  sarcotesta  are  pro- 
vided with  a  system  of  fibrovascular  strands,  but  tracheary  ele- 
ments of  any  kind 
are  absent  in  the 
region  of  the  nucel- 
lus  or  megaspo- 
rangium. 

The  type  of 
seed  delineated  in 
connection  with 
the  foregoing  para- 
graph is  not  the 
only  one  charac- 
teristic of  more 
ancient  plants.  In 
Fig.  1 68  is  shown 
another  category 
of  seed  which, 
although  present- 
ing the  general 
features  of  the 
Paleozoic  type,  is 
characterized  by 
certain  interesting 
and  important 
peculiarities.  The 
integument  in  the 
diagram  is  repre- 


FIG.  168.— Diagram  of  an  ancient  type  of  seed  with 
tracheary  mantle  surrounding  the  gametophyte  (modified 
after  Oliver). 


sented  as  consisting  of  an  outer  softer  sarcotesta  and  an  inner 
resistant  sclerotesta.  It  incloses,  as  in  the  other  type,  the  mega- 
sporangium  or  nucellus,  and  this  is  likewise  provided  with  a 
pollen  chamber.  The  only  important  difference  between  the  seed 
under  discussion  and  that  described  in  the  preceding  paragraph 
is  the  distribution  of  the  fibrovascular  bundles.  In  the  seeds 
of  the  first  type  the  tracheary  strands  are  present  in  both 


228  THE  ANATOMY  OF  WOODY- PLANTS 

sarcotesta  and  sclerotesta,  but  are  lacking  in  the  nucellus  or 
megasporangium.  In  the  seed  now  under  consideration  a  fibro- 
vascular  envelope  surrounds  the  megaspore  and  ends  upwardly 
in  the  pollen  chamber.  It  is  clear  in  the  present  instance  that 
the  tracheary  tissues  invade  the  megasporangium  precisely  as 
they  do  the  microsporangium  of  Ginkgo  and  seed  plants  higher 
in  the  evolutionary  scale.  The  function  of  the  tracheary  tissue 
which  ends  under  the  pollen  chamber  doubtless  was  that  of 
supplying  water  to  facilitate  the  germination  of  the  microspores, 
and  the  fertilizing  movements  of  the  antherozoids  originated 
from  these.  It  seems  clear  that  we  have  in  the  type  of  seed 
'figured  in  connection  with  the  present  paragraph  a  counterpart 
to  microsporangia  with  the  endokinetic  mode  of  dehiscence.  A 
number  of  ancient  seeds  of  the  anatomical  organization  indicated 
here  have  been  investigated,  but  unfortunately  they  have  not  been 
connected  with  absolute  certainty  with  any  definite  vegetative 
types. 

The  seed  of  the  living  Ginkgo  throws  no  light  upon  the  question 
of  the  affinities  of  the  second  type  of  Paleozoic  seeds,  for,  although 
tracheary  tissues  are  abundant  in  the  base  of  the  seed,  they  do  not 
penetrate  into  the  megasporangium  proper.  It  is  likely  that 
degeneracy  of  the  fibrovascular  structures  has  here  obscured  the 
real  situation,  since  from  the  organization  of  the  microsporangium 
in  the  genus  we  should  expect  to  discover  tracheary  elements  in  the 
walls  of  the  nucellus  or  megasporangium. 

In  the  seeds  of  the  Cycadales  we  have  realized  the  general 
features  of  organization  depicted  in  Fig.  167,  which  is,  in  fact, 
modeled  from  the  young  seed  of  Cycas  revoluta.  The  organization 
of  the  microsporangium  in  the  cycads  does  not  support  the  hypoth- 
esis that  nucellar  tracheids  were  once  present  in  the  seed  of  the 
group  and  have  disappeared  in  its  modern  representatives.  The 
conservative  tendencies  of  microsporangial  structures  as  contrasted 
to  those  of  the  megasporangium  or  seed  which  is  considerably 
more  rapidly  influenced  by  the  course  of  evolution  constitute  a 
valuable  situation  from  the  standpoint  of  comparative  morphology. 

The  general  anatomical  features  of  the  seed  in  the  higher  gymno- 
sperms  may  next  be  considered.  In  Fig.  169  is  illustrated  the  more 


THE  MEGASPORANGIUM  AND  SEED 


229 


important  structures  of  the  seed  in  one  of  the  Abietineae  or  pine 
family.  It  is  provided  with  an  integument  in  which  a  small  pore 
known  as  the  micropyle  is  present  at  the  apex  of  the  seed.  Within 
the  integument  is  accommodated  the  nucellus  or  megasporangial 
portion  of  the  seed.  This  is  not  characterized,  as  is  that  of  the 
older  and  fernlike  gymnosperms, 
by  the  presence  of  a  pollen  cham- 
ber. The  pollen  grains  or  micro- 
spores  are  received,  in  fact,  on  the 
smooth  apex  of  the  nucellus  and 
very  soon  send  out  pollen  tubes 
which  bore  their  way  through  the 
tissues  of  the  nucellus  in  order  to 
reach  the  archegonia,  situated  on 
the  apex  of  the  gametophyte. 
The  absence  of  the  pollen  cham- 
ber and  the  presence  of  functional 
pollen  tubes  are  features  which 
most  clearly  distinguish  the  seeds 
of  the  modern  gymnosperms  from 
those  of  the  lower  largely  extinct 
types  included  under  the  heading 
of  the  Archigymnospermae.  The 
nucellus  is  not  provided  with  a 
tracheary  mantle,  such  as  is  found 
in  the  ovular  structures  of  certain 
extinct  gymnosperms.  The  soft 
sarcotesta  often  presented  by 
more  ancient  types  of  seeds  is 

likewise  conspicuous  by  its  absence  in  the  case  of  the  more  typical 
representatives  of  the  higher  gymnosperms. 

In  the  angiosperms  the  situation  in  regard  to  the  seed  is  still 
further  modified  by  the  fact  that  the  pollen  is  no  longer  received  on 
the  apex  of  the  ovule  or  young  seed,  but  comes  to  rest  on  a  special 
region  of  the  closed  sporophyll  known  as  the  stigma  (Fig.  170). 
The  microspores  or  pollen  in  germinating  send  out  pollen  tubes, 
of  greater  or  less  length,  which  penetrate  first  the  tissues  of  the 


FIG.  169. — Diagram  of  seed  of  a  conifer 


230  THE  ANATOMY  OF  WOODY  PLANTS 

closed  reproductive  leaf  or  megasporophyll,  and  later  enter  the  ovule, 
either  through  the  micropyle  (in  porogamous  forms)  or  through 
the  chalaza  or  breech  of  the  seed  (a  condition  found  in  the  so-called 
chalazogamous  dicotyledons).  Not  only  do  inclosure  of  the  ovules 


FIG.  170. — Diagram  of  a  poroga-  FIG.  171. — Diagram  of  a  chalazog- 

mous  dicotyledon.  amous  dicotyledon. 

in  an  ovary  and  the  consequent  exclusion  of  the  pollen  from  direct 
access  distinguish  the  angiospermous  seed  from  that  found  in 
lower  forms,  but  likewise  the  very  considerable  reduction  in  the 
amount  of  gametophytic  tissue.  In  this  large  group  of  seed  plants 
the  prothallial  portion  of  the  young  seed  contains  typically  eight 


THE  MEGASPORANGIUM  AND  SEED 


231 


nuclei,  which  by  the  fusion  of  two  become  seven.  Of  these,  one 
nucleus  surrounds  itself  with  a  protoplasmic  body  to  become  the 
egg,  while  two  others  are  related  to  the  so-called  synergidae.  Of 
the  remaining  four  nuclear  structures,  three  belong  to  the  antipo- 
dals,  a  group  of  cells  present  in  the  base  of  the  embryo  sac  or 
prothallus,  while  the  fourth,  the  product  of  the  fusion  of  two  nuclei 
as  mentioned  above,  becomes  the 
so-called  endosperm  nucleus,  which 
later  develops  the  endosperm  or 
food  substance  of  the  ripened  seed. 
In  certain  of  the  chalazogamous 
angiosperms  (Fig.  171),  notably 
Casuarina  and  the  hazel  (Corylus), 
tracheids  are  found  present  in  the 
nucellus.  The  most  natural  inter- 
pretation of  this  condition  is  in 
connection  with  the  tracheary 
apparatus  in  certain  extinct  seeds 
described  in  a  foregoing  paragraph. 
If  the  presence  of  tracheary  tissues 
in  the  substance  of  the  megaspo- 
rangium  or  nucellus  in  certain 
angiosperms  is  to  be  interpreted  as 
the  persistence  of  an  ancestral  char- 
acter, it  would  indicate  a  relatively 
primitive  position  for  the  chalazog- 
amous forms  in  which  it  occurs. 

The  structure  of  the  mature  seed  in  the  angiosperms  naturally 
claims  a  greater  interest  in  a  work  devoted  to  anatomy.  We  may 
first  take  the  cases  of  dicotyledons  and  monocotyledons.  Fig.  172 
illustrates  the  organization  of  the  seminal  organ  of  the  Indian 
corn  (Zed).  To  one  side  lies  the  embryo,  which  is  provided  with  a 
single  seminal  leaf  or  cotyledon.  This  is  very  large  in  size  and  has 
its  inner  surface  applied  broadly  to  the  food  substance  of  the  seed 
or  endosperm.  The  embryo  proper  lies  to  the  outside  of  the 
cotyledon  and  is  characterized  by  the  presence  of  the  primary  shoot, 
or  plumule,  and  the  primary  root,  or  radicle.  These  are  inclosed 


FIG.  172. — Seed  of  Zea  mais 


232 


THE  ANATOMY  OF  WOODY  PLANTS 


in  protective  sheaths  respectively  known  as  coleoptyle  and  cole- 
orhiza.  The  endosperm  and  embryo  are  inclosed  by  a  covering 
composed  of  the  fused  wall  of  the  ovary  and  the  integuments  of 
the  ovule.  On  the  flatter  side  of  the  seed  toward  which  the  embryo 


FIG.  173. — Seed  of  Celastrus  scattdens 


FIG.  174. — Seed  of 
Pinus  palustris. 


is  placed  a  more  or  less  prominent  elevation  is  seen  which  is  the 
base  of  the  style. 

Fig.  173  is  a  photomicrograph  of  a  dicotyledonous  seed.  Here 
the  embryo  occupies  a  median  position  and  is  provided  with 
two  cotyledons  instead  of  a  single  one.  The  endosperm  or  food 
substance,  as  a  result  of  the  position  of  the  embryo,  entirely  sur- 
rounds the  rudiment  of  the  young  plant.  In  the  seed  illustrated 
the  coat  consists  of  the  hardened  integument,  and  the  wall  of  the 
ovary  is  not  involved  in  the  formation  of  the  seminal  covering. 


THE  MEGASPORANGIUM  AND  SEED  233 

The  typical  condition,  of  course,  for  both  monocotyledons  and 
dicotyledons  so  far  as  the  coat  or  coats  of  the  seed  are  concerned  is 
that  shown  in  Fig.  173,  since  only  rarely  does  the  wall  of  the  ovary 
participate  in  the  formation  of  the  protective  envelope. 

Certain  other  varieties  are  presented  by  the  organization  of  the 
seeds  of  angiosperms.  For  example,  the  endosperm  or  food  sub- 
stance may  be  absent  altogether,  a  condition  illustrated  by  the 
legumes  and  the  Compositae  among  the  dicotyledons  and  by  the 
Orchidaceae  among  the  monocotyledons.  Again,  the  nucellar  or 
megasporangial  substance,  usually  absorbed  as  the  development 
of  the  endosperm  and  embryo  proceeds,  sometimes  persists  and  is 
then  known  as  perisperm.  Another  variation  which  may  present 
itself  is  the  development  of  a  supernumerary  integument,  often 
brightly  colored,  after  the  seed  has  been  fertilized.  This  subsidiary 
coat  is  known  as  the  arillus  and  is  frequently  found  in  families  not 
nearly  related  systematically. 

The  organization  of  the  seed  in  the  pine  is  presented  in  Fig.  1 74 
for  comparison  with  the  angiospermous  conditions  illustrated  in 
Fig.  173.  The  integument  is  clearly  distinguished  as  a  hard  in- 
vestment surrounding  the  abundant  endosperm.  The  food  sub- 
stance in  the  seeds  of  the  gymnosperms  is  derived  directly  from  the 
transformation  of  the  gametophyte  and  is  not  a  new  structure,  as 
is  the  case  with  the  endosperm  of  angiospermous  seeds.  Within 
the  substance  of  the  endosperm  lies  the  embryo,  with  its  narrower 
end  toward  the  micropylar  region  of  the  integument.  The  more 
slender  portion  of  the  embryo  or  young  sporophyte  is  the  primary 
root,  which  is  capped  with  more  or  less  lax  tissues.  These  are  the 
remains  of  the  suspensors  which  in  the  developing  seminal  organ 
forced  the  young  embryo  down  into  the  midst  of  the  endosperm. 
The  larger  end  of  the  embryonic  sporophyte  owes  its  breadth  to  the 
presence  of  numerous  cotyledons  or  seed  leaves,  and  these  dis- 
tinguish the  embryo  of  the  pine  from  those  of  the  angiosperms. 


CHAPTER  XVII 
THE  CANONS  OF  COMPARATIVE  ANATOMY 

With  the  completion  of  the  consideration  of  the  various  tissues 
and  organs  we  are  in  a  position  to  take  up  the  relation  of  anatom- 
ical structure  to  evolutionary  sequence  in  the  various  groups  of 
vascular  plants.  Before  we  proceed  to  this  phase  of  the  subject, 
however,  it  will  be  necessary  to  consider  the  general  principles  or 
canons  of  comparative  anatomy.  It  may  be  pointed  out  in  this 
connection  that  anatomy,  in  common  with  other  branches  of  the 
sciences,  is  based  on  inductive  reasoning.  The  general  principles 
are  consequently  arrived  at  as  the  result  of  the  consideration  of 
large  numbers  of  facts  with  particular  regard  to  the  conclusions 
which  may  be  drawn  from  them.  The  anatomy  of  plants  has 
made  great  progress  in  recent  years  and  in  direct  proportion  to 
our  increasing  knowledge  of  fossil  forms.  The  most  interesting 
and  valuable  results  from  the  evolutionary  standpoint  have  been 
reached  in  connection  with  the  anatomical  investigation  of  extinct 
organisms  of  earlier  geological  ages.  Naturally  those  of  the  great 
coal-producing  period,  the  Paleozoic,  first  received  attention  on 
account  of  the  importance  of  the  study  of  plants  of  that  age  in 
connection  with  the  search  for  productive  coal  seams.  In  more 
recent  years  the  Mesozoic,  which  is  of  the  greatest  interest  in 
relation  to  the  appearance  of  our  modern  types,  has  begun  to  be 
studied.  The  results  bearing  on  the  advancement  of  our  knowledge 
of  the  general  principles  of  the  evolution  of  plants  exemplified  in 
their  anatomical  structure  cannot  be  too  highly  estimated.  It  will 
be  the  aim  of  the  present  chapter  to  set  forth  comprehensively  the 
main  conclusions  of  anatomical  paleobotany  in  their  relation  to  the 
interpretation  of  the  affinities  of  the  main  groups  of  vascular  plants 
now  in  existence  on  the  earth. 

THE  DOCTRINE   OF   RECAPITULATION 

An  important  general  doctrine  developed  in  connection  with 
the  evolutionary  study  of  living  beings  is  the  hypothesis  of  recapitu- 


THE  CANONS  OF  COMPARATIVE  ANATOMY  235 

lation.  It  is  assumed  in  connection  with  this  doctrine  that  the 
young  of  any  species  may  hi  the  course  of  its  individual  development 
pass  through  the  phases  present  in  ancestral  forms.  As  examples 
of  this  principle  we  may  take  extreme  types  of  vegetation,  such  as 
the  almost  leafless  cacti,  forms  with  phylloclads,  or  those  character- 
ized by  the  presence  of  short-shoots.  In  the  mass  of  Cactaceae  the 
leaves  are  abortive  and  are  represented  at  most  by  spines.  In  the 
seedlings,  in  contrast  to  the  adult, 
the  foliar  organs  are  distinctly  pres- 
ent and  are  clearly  recognizable  as 
such.  In  coniferous  species  belong- 
ing to  the  genus  Phyllodadus  the 
branches  in  the  mature  state  form 
flattened  expansions  known  as 
phylloclads.  If  a  seedling  of  any 
species  of  Phyllodadus  be  exam- 
ined, it  becomes  clear  that  a  nor- 
mal round  and  leafy  axis  is  present 
such  as  ordinarily  characterizes  the 
conifer  (Fig.  175).  Likewise  hi 
the  pine  the  seedling  shows  the 
primary  leaves  arranged  on  the  FlG'  175. -Seedlings  and  mature 

.  i    r       i  .          r  branch  of  Phyllodadus  species. 

stem  in  the  usual  fashion  for 

coniferous  gymnosperms  and  not  clustered  on  short-shoots  or 
brachyblasts  as  in  the  adult  branches  of  the  genus.  Further, 
hi  a  conifer  like  the  larch,  which  is  differentiated  in  habit  from  the 
mass  of  the  group  by  its  deciduous  foliage,  we  find  in  the  seedling 
that  the  leaves  persist  for  several  years,  thus  revealing  the  probable 
ancestral  condition  for  the  genus.  An  additional  example  among 
the  dicotyledons  is  supplied  by  the  oak.  The  adult  in  north- 
ern oaks  is  characterized  by  deciduous  leaves.  Oak  seedlings  and 
saplings,  however,  even  in  the  case  of  typically  northern  species, 
retain  their  leaves  during  the  winter,  thus  recalling  a  situation 
characteristic  of  the  live  oaks  of  warmer  latitudes  which  have 
evergreen  foliage  and  represent  anatomically  the  primitive  type 
of  organization. 


236  THE  ANATOMY  OF  WOODY  PLANTS 

The  phenomenon  of  recapitulation  is  not  confined,  however,  to 
external  features  of  organization,  for  it  is  often  equally  well  exempli- 
fied by  internal  anatomical  structure.  A  good  illustration  of  the 
principle  of  recapitulation  is  presented  by  the  seedling  of  the 
araucarian  conifers.  The  adult  stem  of  the  kauri,  or  of  any  other 
araucarian  conifer,  is  characterized  by  two  unique  features.  One 
of  these  is  the  persistence  of  the  traces  belonging  to  the  leaves  long 
after  the  foliar  organs  have  fallen.  The  foliar  fibro vascular  strands 
are  continued  for  many  years,  amounting  in  some  cases  to  centuries, 
through  the  activity  of  the  cambium,  even  when  the  surface  of  the 
trunk  has  long  ceased  to  show  even  the  scars  of  the  leaves  of  which 
they  were  once  the  fibrovascular  supply.  Further,  in  araucarian 
woods  there  is  present  a  peculiar  variety  of  tracheary  structure 
which  clearly  differentiates  their  ligneous  organization  from  that  of 
all  other  living  subtribes  of  conifers.  The  tracheids  in  the  Arau- 
cariineae  have  their  bordered  pits  arranged  in  an  alternating  manner 
and  not  disposed  in  an  opposite  fashion,  as  is  the  characteristic 
condition  in  the  rest  of  the  living  conifers.  In  araucarian  woods  of 
the  Mesozoic  belonging  to  the  genus  Brachyoxylon  the  leaf  traces 
persist  only  for  a  short  time  and  are  no  longer  formed  through  the 
instrumentality  of  the  cambium  after  the  leaves  to  which  they 
belong  have  fallen  from  the  stem.  Again,  the  pits  do  not  manifest 
the  alternating  and  crowded  condition  presented  by  the  wood  of  the 
living  genera.  In  the  seedling  of  both  Agathis  and  Araucaria 
the  leaf  trace  persists  only  so  long  as  it  is  related  to  a  functional 
leaf,  and  does  not  continue  to  develop  for  many  years  after  the  fall 
of  the  foliar  organs,  as  is  the  case  in  the  older  trunk.  Also  in  the 
araucarian  seedling  the  pitting  is  like  that  found  in  the  Cretaceous 
araucarian  genus  Brachyoxylon.  In  this  instance  we  have  a  striking 
exemplification  of  the  principle  of  recapitulation. 

The  law  or  principle  under  consideration  has  many  illustrations 
in  the  vascular  plants,  but  on  the  whole  it  cannot  be  said  to  have 
so  great  a  validity  as  among  the  higher  animals.  It  is  further 
necessary  to  note  in  the  present  connection  that  the  absence  of  a 
given  structure  in  young  individuals  is  by  no  means  evidence 
of  its  absence  in  the  ancestral  forms  from  which  they  have  been 
derived.  For  example,  there  is  good  evidence  that  the  cycadean 


THE  CANONS  OF  COMPARATIVE  ANATOMY      237 

gymnosperms  have  come  from  ancestors  possessing  concentric 
bundles  and  centripetal  wood,  yet  the  seedlings  of  cycads  in  general 
do  not  support  this  conclusion  by  their  anatomical  organization. 
The  doctrine  of  recapitulation  is  of  value,  accordingly,  when  it 
presents  positive  evidence  from  the  seedling  for  the  ancestral 
occurrence  of  a  given  feature  of  organization;  but  negative  testi- 
mony from  this  standpoint  must  be  estimated  as  having  little  or 
no  value.  A  failure  to  realize  this  situation  is  responsible  for  much 
fallacious  biological  reasoning. 

A  very  important  exemplification  of  recapitulation  is  frequently 
supplied  by  the  first  annual  ring  of  the  older  stem  of  arboreal  forms. 
Often  in  groups  which  have  suffered  considerable  reduction,  such 
as,  for  example,  the  gymnosperms  in  general,  the  phenomenon  of 
recapitulation,  although  absent  in  the  seedling,  may  be  clearly 
illustrated  by  the  first  annual  increment  of  woody  growth  in  the 
older  regions  of  the  stem.  An  illustration  of  this  principle  is 
supplied  by  the  living  araucarian  conifers.  Taking  as  an  example 
the  genus  Agathis,  the  kauri  of  Australasia  and  the  East  Indian 
region,  we  find  in  the  first  annual  ring  an  organization  distinctly 
different  from  that  in  the  subsequent  annual  increments  of  the 
wood.  More  or  less  abundant  wood  parenchyma  is  present, 
although  longitudinal  storage  elements  are  conspicuous  by  their 
absence  in  the  adult  wood  of  the  stem.  This  situation  is  of  great 
interest  in  view  of  the  fact  that  the  fossil  wood  of  the  kauri  from 
American  Cretaceous  deposits  is  characterized  by  the  presence  of 
parenchymatous  cells,  not  only  in  the  first  annual  ring,  but  in  all 
subsequent  zones  of  ligneous  growth.  The  persistence  of  the 
structure  of  Mesozoic  forms  in  the  first  annual  ring  of  living  species 
of  the  genus  Agathis  is  a  feature  most  appropriately  falling  under 
the  principle  of  recapitulation.  The  situation  here  indicated  is  of 
great  value  and  wide  validity,  not  only  for  the  gymnosperms,  but 
also  for  the  dicotyledons.  It  might  readily  be  much  more  abun- 
dantly exemplified  in  the  present  connection,  but  the  instance 
supplied  above  will  serve  to  make  the  situation  clear.  Many  other 
cases  will  present  themselves  in  later  chapters  in  connection  with 
the  discussion  of  the  evolution  of  the  different  groups  as  inferred 
from  their  anatomical  organization. 


238  THE  ANATOMY  OF  WOODY  PLANTS 

THE  DOCTRINE   OF  CONSERVATIVE   ORGANS 

This  doctrine  has  received  a  great  impetus  from  the  study  of 
Mesozoic  conifers,  but  was  first  put  forward,  naturally  enough,  in 
connection  with  comparisons  between  the  older  existing  gymno- 
sperms  and  their  Paleozoic  ancestors.  The  leaf  first  came  into 
prominence  in  relation  to  the  hypothesis  of  conservative  organs. 
It  has  been  known  for  many  years,  particularly  since  the  investiga- 
tions of  Mettenius,  that  the  foliar  organs  of  the  Cycadales  present 
remarkable  features  of  anatomical  structure.  Here  the  fibro- 
vascular  bundles  of  the  leaves  are  distinguished  by  the  presence  of 
centripetal  or  cryptogamic  wood,  a  detail  of  organization  conspicu- 
ously absent  in  the  stem  of  the  genera  of  the  living  Cycadales.  In 
the  Cycadofilicales  of  the  Paleozoic  the  bundles  of  the  stem  were 
always  characterized  by  the  presence  of  centripetal  xylem  and 
sometimes  by  concentric  organization  as  well.  The  Cycado- 
filicales are  by  common  and  competent  consent  regarded  as  the 
ancestral  types  from  which  the  living  cycads  have  been  derived. 
The  clear  and  universal  presence  of  centripetal  wood  in  the  foliar 
fibrovascular  bundles  of  living  genera  of  the  Cycadales  is  good 
evidence  at  once  of  the  relationship  of  these  gymnosperms  to  the 
Cycadofilicales  and  of  the  validity  of  the  doctrine  of  conservative 
organs  so  far  as  it  applies  to  the  anatomy  of  leaves.  Many  other 
illustrations  of  the  prevalence  of  this  principle  might  be  supplied 
in  foliar  organs,  but  the  one  described  above  will  serve  appropri- 
ately to  elucidate  the  situation  and  is  particularly  apposite  in  the 
present  connection  because  it  is  probably  the  first  case  to  be  cited 
in  evolutionary  anatomy. 

The  foliar  organs  are  not,  however,  the  only  parts  of  the  adult 
plant  which  present  illustrations  of  the  principle  of  conservative 
organs.  The  stem  in  that  region  devoted  to  the  function  of  repro- 
duction has  also  figured  strikingly  in  this  connection.  To  Scott 
belongs  the  credit  of  having  drawn  attention  to  the  fact  that  the 
peduncle  or  base  of  the  cone  in  certain  Cycadales  furnishes  a  clear 
example  of  the  persistence  of  an  ancestral  structure  in  the 
reproductive  axis  which  has  quite  disappeared  in  the  ordinary 
vegetative  stem.  In  Stangeria,  Zamia,  and  other  genera  of  the 
Cycadales  he  noted  that  the  fibrovascular  strands  of  the  axes 


THE  CANONS  OF  COMPARATIVE  ANATOMY      239 

of  the  cones  frequently  manifested  the  presence  of  vestiges  of 
centripetal  wood,  although  xylem  belonging  to  this  category  has 
wholly  disappeared  in  the  vegetative  axis.  The  value  of  this 
generalization  can  scarcely  be  overestimated.  During  the  interval 
of  nearly  twenty  years  dating  from  Scott's  brilliant  discovery, 
much  additional  evidence  has  been  supplied  in  support  of  the 
conservatism  of  the  anatomical  structure  of  the  reproductive  axes. 
For  example,  it  has  been  shown  by  comparison  of  the  vegetative 
organization  of  Mesozoic  conifers  with  the  anatomical  structures 
found  in  the  ovuliferous  cones  of  living  forms  that  the  latter  fre- 
quently perpetuate  features  which  have  vanished  in  the  vegetative 
parts.  This  is  manifestly  the  case  in  the  abietineous  and  arauca- 
rian  conifers  which  compete  with  each  other  for  the  claim  of  being 
the  most  ancient  representative  of  the  coniferous  stock.  The  cone 
in  both  Pinus  and  Agathis  presents  numerous  Mesozoic  features 
which  have  disappeared  elsewhere  in  the  stem  organs  of  existing 
species  belonging  to  these  genera. 

The  value  of  the  anatomy  of  reproductive  axes  cannot  be 
estimated  so  highly  in  the  case  of  the  angiosperms,  since  the 
relatively  slight  development  of  fibrovascular  structures  in  flowers 
and  inflorescences  leaves  less  scope  for  the  appearance  of  phylo- 
genetically  significant  structures.  This  situation  needs  to  be 
particularly  emphasized  in  view  of  some  recent  highly  fallacious 
attempts  to  utilize  the  anatomy  of  reproductive  axes  in  working 
out  evolutionary  sequences  in  the  woody  dicotyledons.  Clearly  in 
this  instance  only  structures  are  significant  which  find  adequate 
development  in  the  somewhat  slender  annual  woody  cylinder  of  the 
flowering  parts  of  perennial  dicotyledons.  In  the  monocotyledons 
the  restrictions  on  interpretation  are  still  greater  on  account  of  the 
usual  absence  of  secondary  growth  in  this  great  division  of  the 
angiosperms.  In  the  application  of  the  doctrine  of  conservative 
parts  to  the  reproductive  axes  of  the  angiosperms  it  must  be 
recognized  that  a  little  knowledge  is  a  dangerous  thing. 

It  has  been  made  clear  in  the  preceding  paragraphs  that  the 
doctrine  of  conservative  parts  is  well  exemplified  by  the  leaf  and 
is  manifested  in  the  stem  by  the  more  archaic  axes  connected  with 
reproduction.  The  situation  in  the  organs  cited  has  for  a  long  time 


240  THE  ANATOMY  OF  WOODY  PLANTS 

been  apparent.  Of  more  recent  origin  is  the  realization  that,  in 
modern  plants  at  any  rate,  the  root  is  the  most  valuable  of  all 
organs  from  the  standpoint  of  evolutionary  anatomy.  In  the 
case  of  comparisons  between  Paleozoic  and  Mesozoic  groups  the 
root,  in  fact,  has  proved  to  be  in  general  too  conservative  to  furnish 
significant  examples  of  the  retention  of  ancestral  characters.  The 
search  for  centripetal  or  cryptogamic  wood  has  been  in  the  fore- 
ground in  investigations  bearing  on  the  relationship  between 
Paleozoic  and  Mesozoic  groups.  Since  in  all  forms  of  every  geo- 
logical age  the  root  has  centripetal  primary  wood,  in  regard  to  this 
crucial  feature  of  more  ancient  types  it  obviously  does  not  supply 
distinctive  evidence  in  connection  with  the  doctrine  of  descent. 
In  other  respects,  however,  and  particularly  with  reference  to  the 
organization  of  the  tissues  of  the  secondary  wood,  the  root  has 
proved  itself  to  be  of  greater  significance  than  any  other  organ  of 
the  higher  plants.  Of  course  the  most  striking  evidence  of  con- 
servatism in  the  root  has  resulted  from  the  comparison  of  Mesozoic 
and  modern  forms.  It  has,  for  example,  been  shown  that  the  root 
in  modern  conifers  clearly  and  persistently  perpetuates  the  features 
of  organization  which  characterized  the  structure  of  the  stem  in 
the  Cretaceous  and  earlier  periods  of  the  Mesozoic  age.  In  Pinus 
and  Agathis  the  root  presents  in  many  of  its  earlier  annual  rings 
the  distinctive  organization  found  in  the  stem  of  these  types 
in  the  Mesozoic.  Nor  is  the  inherent  conservatism  of  the  root  of 
value  in  the  study  of  the  gymnosperms  alone.  Although  our 
knowledge  of  the  anatomical  structures  of  the  angiosperms  in 
Mesozoic  times  is  as  yet  extremely  inadequate,  we  are  able  in 
many  instances  by  the  application  of  the  doctrine  of  the  con- 
servatism of  the  anatomical  organization  of  the  root  to  infer  the 
ancestral  type  of  stem  in  the  highest  vascular  plants. 

It  will  be  evident  from  the  foregoing  paragraphs  that  conserva- 
tism is  particularly  inherent  in  the  leaf  and  root  of  vascular  plants 
and  that  the  highly  progressive  stem  presents  only  features  which 
are  of  interest  from  the  standpoint  of  the  doctrine  of  descent,  either 
in  its  first  annual  ring  or  in  axes  specially  allocated  to  the  function 
of  reproduction.  The  sporangium  is  a  fourth  structure  recognized 
as  a  primitive  organ  of  vascular  plants  in  an  earlier  chapter  of  the 


THE  CANONS  OF  COMPARATIVE  ANATOMY  241 

present  work.  Here  the  lack  of  complexity  of  organization  rather 
militates  against  the  presence  of  phylogenetically  important 
structures.  It  must  nevertheless  be  noted  that  the  sporangium  is 
an  extremely  conservative  organ  and,  as  far  as  the  relative  simplicity 
of  its  organization  supplies  points  of  comparison,  is  of  very  great 
significance  for  the  doctrine  of  evolution.  It  has  been  demonstrated 
in  an  earlier  chapter  that  the  sporangium  perpetuates  the  protean 
centripetal  xylem  in  the  form  of  its  opening  mechanism  to  a  higher 
point  than  any  other  organ  but  the  root.  The  value  of  the  spore 
sac  in  phylogeny,  although  limited  by  the  relative  simplicity  of 
the  organ,  must  consequently  be  estimated  as  great. 

The  doctrine  of  conservative  organs  is  of  the  greatest  signifi-  ^ 
cance  in  connection  with  the  study  of  the  evolutionary  history 
of  plants,  because  of  the  abundance  and  reliability  of  the  evidence 
which  the  various  parts  furnish  in  this  connection.  Obviously,  if 
leaf,  reproductive  axis,  root,  and  sporangium  all  supply  consonant 
and  harmonious  testimony  in  the  same  direction,  a  sound  con- 
clusion must  inevitably  be  reached.  There  can  be  little  question 
that  the  doctrine  of  conservative  organs  is  the  most  important  one 
which  modern  inductive  anatomy  has  supplied  as  a  tool  of  evolu- 
tionary investigation.  In  fact,  the  general  principles  included 
under  this  head  are  of  such  great  significance  that  the  present 
volume  may  be  considered  as  written  only  for  those  whose  anatom- 
ical training  has  progressed  to  such  a  point  that  they  are  able  to 
appreciate  the  universal  value  and  validity  of  the  doctrine  here 
discussed. 

THE  DOCTRINE   OF  REVERSION 

This  doctrine  is  well  shown  in  the  case  of  plants  with  a  consider- 
able amount  of  secondary  growth — namely,  the  gymnosperms  and 
the  dicotyledonous  angiosperms.  It  is  of  little  value  in  herba- 
ceous forms,  whether  cryptogamic  or  phanerogamic,  since  in  these 
the  effects  of  injury  are  usually  in  the  direction  of  degeneracy 
only.  In  plants  with  secondary  growth  and  consequently  more 
massive  organs  the  effect  of  injuries  frequently  is  to  recall  ances- 
tral features  of  organization.  This  phenomenon  is  called  reversion. 
Only  in  certain  conifers  can  we  observe  the  effect  of  injury  in 
recalling  in  living  forms  features  which  are  known  to  have  been 


242  THE  ANATOMY  OF  WOODY  PLANTS 

present  in  their  Mesozoic  forebears.  For  example,  in  the  arau- 
carian  conifers,  cited  earlier  in  another  connection,  the  Mesozoic 
organization  of  the  wood  is  recalled  in  the  stem  of  the  existing 
species  as  a  consequence  of  injury.  The  wood  formed  subsequent  . 
to  the  infliction  of  the  injury  shows  a  type  of  structure  charac- 
teristic of  the  group  in  an  earlier  geological  epoch.  Although 
only  in  rare  instances  can  structures  which  are  actually  known 
to  have  been  present  in  the  past  be  recalled  by  injuries,  in  many 
other  cases  we  find  the  infliction  of  wounds  followed  by  the  for- 
mation of  features  which  are  normally  present  in  the  conserva- 
tive organs.  Consequently  the  doctrine  of  reversion  finds  support, 
not  only  in  the  facts  of  paleobotanical  anatomy,  but  also  in  the 
much  more  richly  exemplified  doctrine  of  conservative  parts.  It 
is  further  clear  that  we  cannot  interpret  all  structural  peculiarities 
which  result  from  injury  as  reversions  to  a  former  condition  of 
organization,  but  only  those  which  are  definitely  paralleled  by 
known-  conditions  in  fossil  forms  or  are  illustrated  in  the  anatomy 
of  the  conservative  organs — the  leaf,  reproductive  axis,  and  stem, 
or  at  least  some  of  these. 

Carefully  as  the  doctrines  of  recapitulation  and  of  conservative 
parts  must  be  applied  to  elucidate  the  course  of  evolution,  the 
principle  of  reversion  must  be  invoked  in  phylogenetic  studies 
with  even  greater  precautions.  A  wide  knowledge  of  fossil  forms 
as  well  as  an  extensive  acquaintance  with  the  facts  of  comparative 
anatomy  are  necessary  for  a  successful  application  of  this  doctrine 
to  the  data  derived  from  the  investigation  of  injuries.  In  this 
connection  it  is  further  necessary  to  distinguish  clearly  between 
the  phenomena  of  hypertrophy  and  those  of  actual  reversion.  For 
example,  when  the  stern  of  a  dicotyledon  or  of  a  conifer  is  injured,  a 
local  damming  of  food  substances  results  as  a  consequence  of  the 
elimination  of  the  conducting  tissues  of  the  phloem  in  the  immediate 
vicinity  of  the  wound.  Overdevelopment  consequent  on  over- 
nutrition  accordingly  is  locally  most  conspicuous.  Very  often 
true  reversionary  changes  resulting  from  the  impulse  supplied  by 
the  injury  present  themselves  only  at  some  distance  from  the 
wound.  This  situation  is  found  in  the  conifers  in  connection  with 
reversionary  conditions  following  injuries  in  the  case  of  the  rays 


THE  CANONS  OF  COMPARATIVE  ANATOMY  243 

which  ordinarily  manifest  themselves,  not  in  the  actual  wound  cap 
or  hypertrophied  mass  of  wood  formed  after  injury,  but  on  the 
opposite  side  of  the  stem.  Among  the  dicotyledons  the  birch 
exemplifies  the  same  condition  in  regard  to  the  formation  of  aggre- 
gate rays.  The  diffuse  structure  of  radial  parenchyma  which 
characterizes  the  normal  organization  of  the  wood  of  the  genus 
Betula  gives  way  to  a  condition  of  aggregation  following  injury,  not 
on  the  edges  of  the  wound  (that  is,  in  the  actual  wound  cap),  but 
in  the  region  of  the  axis  diametrically  opposite  to  the  injury.  In 
accordance  with  the  general  reversionary  principles  here  stated, 
the  wound  cap  as  often  as.  not  presents  structures  in  an  advanced 
or  accelerated,  and  not  in  a  reversionary,  condition.  Some  recent  ! 
publications  on  the  anatomy  of  the  dicotyledons  reveal  a  failure  to 
realize  this  fundamental  principle,  and  therefore  it  is  well  that  it 
should  be  emphasized  in  the  present  connection. 

The  foregoing  paragraphs  elucidate  the  most  important  canons 
or  principles  of  evolutionary  anatomy.  It  cannot  be  too  strongly 
urged  that  all  the  evidence  avaikble  under  the  various  principles 
here  described  and  exemplified  should.be  brought  into  considera- 
tion, as  a  failure  to  check  up  one  kind  of  evidence  against  another 
often  results  in  a  fallacious  and  ephemeral  deduction.  The  most 
cogent  testimony  to  the  validity  of  any  evolutionary  conclusion  is 
naturally  supplied  by  the  conditions  actually  realized  in  ancestral 
fossil  forms.  Since,  however,  by  reason  of  the  incompleteness  of 
the  geological  record  and  our  consequent  ignorance  of  the  organiza- 
tion of  older  forms  we  are  frequently  not  in  the  position  to  avail 
ourselves  of  the  actual  past  history  of  given  plant  types,  we  must  of 
necessity  have  recourse  to  the  valuable  aid  furnished  by  the 
important  general  principles  described  in  the  earlier  paragraphs 
of  the  present  chapter.  Where  the  canons  of  evolutionary  anatomy 
are  judiciously  employed,  the  result  is  usually  so  clear  and  convin- 
cing as  to  commend  itself  to  the  unprejudiced  mind. 


CHAPTER  XVIII 
THE  LYCOPSIDA  AND  PTEROPSIDA 

The  discussion  of  tissues  and  organs  and  the  elucidation  of  the 
general  principles  applicable  to  these  in  the  earlier  chapters  of  the 
present  work  bring  us  to  the  point  where  the  higher  plants  may 
profitably  be  discussed  in  regard  to  their  general  anatomy  and 
evolutionary  affinities.  Obviously,  if  it  is  possible  to  compass  a 
general  grouping  which  will  at  the  outset  indicate  the  main  lines 
of  evolution,  the  consideration  of  the  particular  groups  will  be 
much  facilitated.  Some  years  ago  the  writer  put  forward  a  general 
classification  of  vascular  plants,  based  on  the  cardinal  features  of 
the  reproductive  structures  and  the  salient  anatomical  characters. 
This  attempt  to  group  the  higher  plants  in  accordance  with  their 
more  important  reproductive  and  anatomical  features  has  met  with 
approval  among  competent  judges  and  as  a  consequence  may  be 
conveniently  utilized  in  the  present  connection. 

A  general  survey  of  vascular  plants,  existing  and  extinct, 
reveals  the  fact  that  there  are  certain  features  correlated  in  a 
significant  way.  For  example,  a  large  number  are  characterized 
by  the  possession  of  ventral  or  adaxial  sporangia  together  with 
usually  small  leaves.  Another  large  assemblage  presents  relatively 
large  leaves  and  more  numerous  sporangia  which  are  dorsal  or 
abaxial  in  position.  The  former  group  is  characteristically  repre- 
sented by  the  lycopods  or  club  mosses  and  their  allies,  while  the 
latter  includes  the  ferns  and  the  forms  more  nearly  related  to  them, 
the  gymnosperms,  and  the  angiosperms.  The  first  aggregation 
of  types,  known  as  the  Lycopsida,  is  now  practically  extinct,  but 
played  a  large  role  in  the  Paleozoic  and  furnished  a  considerable 
proportion  of  the  raw  material  of  that  extremely  important  com- 
bustible called  coal,  together  with  its  derivative  products,  petroleum 
and  natural  gas.  The  second  alliance,  to  which  the  appellation 
Pteropsida  is  appropriately  given,  although  abundantly  present 
in  the  remote  past,  still  prevails  and  has  given  rise  to  the  seed  plants 

244 


THE  LYCOPSIDA  AND  PTEROPSIDA  245 

and  to  the  forests  of  Mesozoic  and  Cenozoic  times,  as  well  as  to 
the  herbaceous  types  which  under  modern  conditions  more  and 
more  predominate  in  the  plant  population  of  our  earth. 

A  more  detailed  characterization  of  the  Lycopsida  is  now  desir- 
able. Fig.  176  represents  diagrammatically  the  organization  of 
an  axis  of  the  lycopsid  type  together  with  its  appendages.  The 
latter  consist  of  leaf,  branch,  and  root.  Root  and  branch  are 
intimately  associated,  the  former  usually  proceeding  from  the  base 
of  the  latter.  The  upper  end  of  the  main  axis  is  represented  in 
transverse  section  to  show  the  anatomical  relations  of  the  organs. 
The  fibrovascular  cylinder  in  the  particular  case  illustrated  is 
siphonostelic,  although  it  might  equally  well  be  protostelic,  espe- 
cially in  the  lower  representatives  of  the  group.  The  tubular 
stele  or  central  cylinder  constituted  by  the  fibrovascular  sys- 
tem is  interrupted  only  at  one  point — where  the  fibrovascular 
supply  of  a  branch  takes  its  origin  from  the  main  cylinder.  The 
gap  thus  appearing  is  known  as  the  branch  gap  or  ramular 
lacuna.  On  the  margin  of  the  stem  appear  certain  projections,  the 
leaf  bases,  within  which  are  included  the  fibrqvascular  strands 
destined  for  the  leaves  or  foliar  traces.  These  subtend  projections 
on  the  surface  of  the  central  cylinder  occupied  by  groups  of  proto- 
xylem.  The  projections  in  question  are  the  starting-points  of  the 
foliar  traces,  and  it  is  clear  that  in  no  instance  is  there  an  inter- 
ruption in  the  continuity  of  the  central  cylinder  corresponding  to 
the  departing  leaf  traces.  The  central  cylinder  of  the  Lycopsida 
is  said  on  this  account  to  be  without  foliar  gaps.  The  absence  of 
leaf  gaps  or  foliar  lacunae  is  characteristic  of  all  the  Lycopsida  and 
is  a  diagnostic  feature  of  importance  for  the  great  group  or  phylum 
which  they  represent.  Even  in  those  Lycopsida  in  which  the 
leaves  are,  superficially  at  least,  relatively  large  (for  example,  the 
Sigillariae,  in  which  the  foliar  organs  were  sometimes  over  a  meter 
in  length)  there  were  still  no  foliar  gaps.  The  absence  of  foliar  gaps 
is  an  anciently  inherited  or  palingenetic  feature  of  the  Lycopsida, 
and  they  may  therefore  be  technically  characterized  as  palin- 
genetically  microphyllous  (small-leaved). 

Not  only  has  this  large  group  universally  small  leaves,  but  it 
also  possesses  another  important,  salient,  and  constant  general 


246  THE  ANATOMY  OF  WOODY  PLANTS 


FIG.  176. — Diagrammatic  representation  of  the  Lycopsida 


THE  LYCOPSIDA  AND  PTEROPSIDA  247 

feature  of  organization — in  this  instance  related  to  reproduction. 
In  the  phylum  under  discussion  the  sporangia  or  spore  sacs  are 
invariably  on  the  upper  or  adaxial  (ventral)  surface  of  the  sporo- 
phyll  or  reproductive  leaf.  The  sporangia  are  single  or  at  most 
relatively  few  in  number  and  are  invariably  ectokinetic  in  their 
mode  of  dehiscence.  True  seeds  seem  never  to  have  made  their 
appearance  in  the  Lycopsida,  although,  as  has  been  pointed  out 
hi  an  earlier  chapter,  structures  somewhat  simulating  seeds  have 
been  found  in  certain  of  the  Paleozoic  representatives  of  the 
group.  The  organs  in  question,  however,  lacked  a  true  integu- 
ment and,  so  far  as  is  known,  were  without  arrangements  for 
receiving  the  microspores  or  pollen  grains,  a  universal  equipment 
in  the  case  of  pteridospermous  and  gymnospermous  seeds.  Possibly 
the  failure  to  achieve  true  seminal  structures  was  the  cause  of  the 
decline  of  the  Lycopsida,  which  at  the  present  time  constitute  an 
insignificant  proportion  of  the  vegetation  of  the  world.  The  group 
is  very  ancient  and  goes  back  to  the  beginning  of  the  geological 
record.  The  primitive  forms  representing  the  Lycopsida  were 
often  arboreal  in  their  habit,  and  the  alliance  reached  its  culmina- 
tion in  the  carboniferous  forests  of  the  Paleozoic.  It  became  largely 
reduced  in  the  Mesozoic,  and  the  Cenozoic  saw  its  virtual  extinction. 
In  the  Pteropsida  we  have  to  do  with  forms  in  which  the  leaves 
are  relatively  large  in  comparison  to  the  stem  and  often  extremely 
complicated  in  structure.  As  is  shown  in  the  diagram  (Fig.  177), 
the  transverse  section  of  the  stem  reveals  a  central  cylinder 
which,  when  siphonostelic,  as  in  the  illustration,  is  characterized 
by  gaps  corresponding  to  the  traces  departing  to  the  leaves.  This 
feature  of  its  anatomy  is  in  marked  contrast  to  that  found  univer- 
sally in  the  Lycopsida,  in  which  the  leaves  are  not  related  to  foliar 
gaps.  The  traces  of  the  branches  in  the  Pteropsida  likewise  sub- 
tend gaps  in  the  central  cylinder  (which  may  be  designated  as 
ramular  gaps).  The  fibro vascular  supply  of  the  root  makes  its 
exit  from  the  stele  or  fibrovascular  cylinder  without  causing  any 
gap  or  interruption.  The  roots  are  often,  but  not  invariably, 
related  to  the  bases  of  the  leaves.  The  foliar  organs,  usually  much 
larger  in  size  than  in  the  Lycopsida  and  often  of  great  dimensions 
and  lobed  in  a  complicated  manner,  are  to  be  regarded  as  palin- 


FIG.  177. — Diagrammatic  representation  of  the  Pteropsida 


THE  LYCOPSIDA  AND  PTEROPSIDA  249 

genetically  megaphyllous,  since  their  anatomical  relations  are 
always  characterized  by  the  presence  of  foliar  gaps.  The  repro- 
ductive organs  or  sporangia  in  the  case  of  the  Pteropsida  are  on  the 
lower  or  abaxial  (dorsal)  surface  of  the  leaf,  and  are  often  numerous 
and  complicated  in  structure.  In  the  lower  forms  the  dehiscence 
of  the  spore  sacs  is  ectokinetic,  but  in  the  higher  representatives 
of  the  phylum  the  opening  mechanism  is  derived  from  modified 
transfusion  tissue  (in  turn  derived  from  the  centripetal  wood  of 
the  ectokinetic  and  lower  forms).  Those  Pteropsida  characterized 
by  an  internal  reproductive  mechanism  derived  from  transfusion 
tissue  are  appropriately  designated  as  endokinetic.  The  higher 
members  of  the  series  have  developed  true  seeds  provided  with  an 
integument  and  equipped  with  an  apparatus  either  related  to  the 
seed  itself  or  to  the  seed  leaf  (or  megasporophyll)  for  the  reception 
of  microspores  or  pollen.  Although  developed  in  very  early 
geological  times  in  forms  resembling  ferns,  the  Pteropsida  are  still 
in  full  vigor;  and  in  their  highest  manifestation,  the  angiosperms, 
they  constitute  an  overwhelmingly  large  proportion  of  the  present 
vegetable  population  of  the  earth.  They  have  reached  their 
zenith  of  efficiency  in  the  herbaceous  angiosperms,  which  in  all 
probability  will  supplant  arboreal  angiospermous  types  in  the  not 
very  remote  future.  The  gymnosperms,  and  in  particular  the 
conifers,  were  the  prevailing  Pteropsida  of  the  Mesozoic,  while  in 
the  Paleozoic  age  pteridosperms  (Cycadofilicales)  and  other  lower 
gymnosperms  and  fern  allies  represented  the  group. 

The  Pteropsida  and  Lycopsida  are  distinct  as  far  back  as  the 
geological  record  can  be  perused,  and  there  seems  to  be  little  doubt 
that  they  constitute  two  primitive  stocks  of  vascular  plants. 
They  are  so  clearly  diverse  even  in  their  earliest  manifestations 
that  it  is  difficult  to  picture  how  they  may  have  been  formerly 
connected.  It  is  obvious,  however,  that  the  lycopsid  type  has  not 
been  able  to  cope  with  the  changing  conditions  of  environment  and, 
comparatively  early  in  the  periods  recorded  in  the  rocks,  was 
relegated  to  a  position  of  relative  inferiority.  Whether  this  situa- 
tion was  the  result  of  the  failure  to  achieve  true  seeds  or  is  to  be 
explained  as  the  basis  of  some  fundamental  defect  of  internal 
organization  incapacitating  the  Lycopsida  to  succeed  in  competition 


250  THE  ANATOMY  OF  WOODY  PLANTS 

with  the  large-leaved  Pteropsida  must,  for  the  present  at  any  rate, 
be  left  an  open  question.  Although  the  Lycopsida  were  the  pre- 
dominant constituent  of  the  Paleozoic  forests,  the  Pteropsida  in 
many  cases  have  entered  largely  into  the  composition  of  the  more 
ancient  coals  and  can  often  be  clearly  recognized,  particularly  in  the 
carboniferous  coals  of  the  Middle  Western  states  (Illinois,  Ohio, 
etc.),  as  charred  remains  of  the  axes  and  even  as  pinnae  or  smaller 
segments  of  the  leaves  in  the  form  of  so-called  "Mother  of  Coal." 

After  having  described  the  general  characteristics  of  the  Lycop- 
sida and  Pteropsida,  we  find  it  in  order  to  indicate  the  main  groups 
which  come  under  these  two  great  divisions  of  the  vascular  plants. 
The  Lycopsida  include  two  principal  subdivisions — the  Lycopo- 
diales  and  Equisetales.  The  Lycopodiales  are  characterized  by 
the  alternating  nature  of  their  foliage,  while  the  Equisetales  have 
their  leaves  disposed  in  whorls  on  a  stem  presenting  marked  ridges 
and  furrows.  The  Lycopodiales  are  again  subdivided  into  isos- 
porous  and  heterosporous  families.  Of  the  former  there  are  two — • 
the  Lycopodiaceae  and  the  Psilotaceae.  The  Lycopodiaceae  are 
characterized  by  the  possession  of  well-developed  roots  and  un- 
divided sporangia,  while  in  the  Psilotaceae  the  sporangial  struc- 
tures are  septate,  and  organs  of  the  nature  of  roots  are  entirely 
absent.  The  heterosporous  Lycopodiales  have  in  common  a  foliar 
appendage,  known  as  a  ligule,  which  is  present  on  both  vegetative 
and  reproductive  leaves,  and  in  the  case  of  the  latter  occurs  just 
above  the  insertion  of  the  sporangium.  Of  the  three  families 
presenting  the  phenomenon  of  heterospory  the  first,  the  Selaginel- 
laceae,  are  terrestrial  forms  included  under  a  single  genus,  Selaginella, 
in  which  the  megasporangia  never  produce  more  than  four  spores. 
In  the  second  and  usually  aquatic  family,  the  Isoetaceae,  there  is 
likewise  a  single  genus,  but  the  sporangia  are  provided  with  trans- 
verse processes  known  as  trabeculae,  and  the  megaspores  are 
numerous  in  each  sack.  Lastly,  the  Lepidodendraceae,  including 
the  Sigillariae  and  their  allies,  are  terrestrial  extinct  forms  often  of 
arboreal  habit  and  of  somewhat  diverse  megasporangial  structures. 

The  Equisetales,  as  has  been  indicated  at  the  beginning  of  the 
foregoing  paragraph,  are  distinguished  by  the  whorled  arrangement 
of  their  leaves.  Another  feature  which  they  possess  in  common  is 


THE  LYCOPSIDA  AND  PTEROPSIDA  251 

the  usual  exhibition  of  a  high  degree  of  multiplication  of  the 
sporangia  which  are  often  disposed  on  common  stalks  known  as 
sporangiophores.  The  branches  instead  of  being  truly  axillary  are 
borne  alternately  with  the  leaves  at  the  nodes.  The  Equisetales 
may  conveniently  be  divided  into  three  families — the  Sphenophyl- 
laceae, the  Calamitaceae,  and  the  Equisetaceae — which  are  in  all 
probability  related  to  one  another  in  the  order  indicated  by  their 
enumeration.  The  Sphenophyllaceae  were  forms  in  which  the 
central  cylinder  of  the  stem  was  protostelic.  The  cones  consisted 
of  sporophylls  presenting  various  degrees  of  complication.  In  the 
simplest  forms  the  sporangia  were  inserted  singly  on  the  stalks  or 
sporangiophores  and  were  numerous  for  each  sporophyll.  In  more 
advanced  types  the  sporangia  became  two  or  more  for  each  spo- 
rangiophore.  The  Calamitaceae,  like  the  Sphenophyllaceae,  are 
organisms  entirely  extinct.  They  usually  possessed  the  arboreal 
habit  and  were  invariably  characterized  by  a  siphonostelic  central 
cylinder.  The  sporangiophores  bore  four  sporangia  and  were 
variously  related  to  the  sporophylls.  In  the  Equisetaceae  are 
included  a  number  of  genera,  living  and  extinct,  of  herbaceous 
habit  and  possessing  so  far  as  is  known  a  simplified  type  of  cone  in 
which  sporophylls  are  represented  by  the  sporangiophores  alone. 

In  the  Pteropsida,  marked  by  the  general  features  enumerated 
in  earlier  paragraphs  of  the  present  chapter,  there  are  three  large 
subdivisions — the  Filicales,  the  Gymnospermae,  and  the  Angio- 
spermae.  The  first  of  these,  as  the  name  indicates,  include  the 
fernlike  forms — that  is,  those  in  which  the  reproduction  takes  place 
through  unicellular  bodies  known  as  spores.  In  the  Gymnospermae 
true  seeds  are  present  which  in  every  case  are  equipped  to  receive 
the  microspores;  and  these  after  germination  effect  fertilization 
either  by  means  of  antherozoids  (Archigymnospermae)  or  through 
the  agency  of  pollen  tubes  (Metagymnospermae).  In  the  last  and 
(in  the  present  epoch)  most  important  subdivision  of  the  Pterop- 
sida, the  Angiospermae,  the  pollen  is  received  on  the  apex  of  the 
closed  megasporophyll  and  no  longer  falls  upon  the  seed.  Fertiliza- 
tion is  invariably  by  means  of  a  pollen  tube.  The  habit  of  the 
angiosperms  is  either  arboreal  or  herbaceous,  and  the  fibrovascular 
tissues  show  a  high  degree  of  specialization. 


CHAPTER  XIX 
THE  LYCOPODIALES 

This  group,  as  has  been  indicated  in  the  last  chapter,  has  spiral 
phyllotaxy.  It  includes  both  isosporous  and  heterosporous 
families.  The  latter  are  distinguished  by  the  presence  of  a  ligule, 
while  in  the  former  this  structure  is  lacking.  In  the  genus  Lyco- 
podium  the  central  cylinder  of  the  stem  is  radial  and  protostelic. 
As  a  consequence  of  the  radial  organization  of  the  stele,  the  masses  of 
phloem  lie  in  the  intervals  between  the  generally  radially  directed 
oands  of  xylem.  The  sieve  tubes  are  separated  from  the  tracheids 
by  several  rows  of  parenchyma  on  either  side.  In  the  vertically 
directed  reproductive  axes  of  Lycopodium  the  organization  of  the 
fibrovascular  tissues  is  typically  radial,  while  in  the  creeping  stems 
or  rootstocks  the  arrangement  of  the  xylem  and  phloem  is  somewhat 
dorsiventral.  The  upright  axes  of  Lycopodium,  aside  from  the 
fact  that  they  bear  leaves,  are  scarcely  distinguishable  in  structure 
from  the  roots.  This  resemblance  in  organization  between  root 
and  shoot  is  an  indication  of  the  antiquity  of  the  lycopodineous 
stock,  since  in  the  higher  groups  the  differentiation  between  the 
axial  and  radical  organs  becomes  more  and  more  marked. 

The  monotypic  genus  Phylloglossum  possesses  a  siphonostelic 
stem.  In  this  form  the  lower  region  of  the  axis  is  tuberous  and 
contains  the  most  massive  development  of  the  fibrovascular  system. 
In  the  tuber  also,  as  is  shown  in  Fig.  178,  there  is  both  internal 
phloem  and  internal  endodermis.  As  the  stele  of  the  inferior 
region  passes  upward,  it  gives  off  a  branch  into  the  peduncle  of 
the  tuber  which  is  to  perpetuate  the  plant  in  a  subsequent  season. 
The  trace  of  this  appendage  in  departing  from  the  central  cylinder 
leaves  a  well-marked  branch  gap.  The  foliar  organs  indicated  as 
swellings  on  the  outline  of  the  section  cause  no  interruptions  in  the 
continuity  of  the  central  cylinder  by  their  departure  from  its  surface. 
In  this  respect  Phyllcglossum  shows  itself,  to  be  a  veritable  repre- 
sentative of  the  Lycopsida.  In  the  higher  and  aerial  region  of  the 

252 


THE  LYCOPODIALES 


253 


FIG.  178. — Diagram  of  the  lower  region  of  the  stem 
in  Phylloglossum  (after  Bertrand). 


axis  the  fibrovascular  tissues  become  so  much  reduced  that  internal 
phloem  is  no  longer  developed,  and  the  continuity  of  the  cylinder 
is  interrupted  by  gaps  which  are  not  related  to  organs,  but  merely 

indicate  the  incom-     */•„ ^ 

plete  development 
of  the  xylem . 
Where  a  trace  is 
given  off,  as  is 
shown  in  Fig.  179, 
it  takes  its  origin 
opposite  a  strand 
and  does  not  sub- 
tend an  interval  be- 
tween the  bundles, 
clearly  showing 
the  lycopsid  condition,  even  in  the  state  of  stelar  reduction  pre- 
sented in  the  evan- 
escent aerial  axis. 

The  Psilotaceae  are 
anatomically  distin- 
guished from  the  Lyco- 
podiaceae  by  the 
absence  of  true  roots. 
Here  the  aerial  stem, 
unless  it  be  of  very 
small  size,  is  siphono- 
stelic  in  its  organiza- 
tion. A  thick- walled 
medulla  is  often  pres- 
ent, but  no  internal 
phloem  has  been  ob- 
served. The  organiza- 
tion of  the  conducting 
tissues  is  radial  and  exarch;  the  leaf  traces,  as  in  Lycopodium,  take 
their  origin  from  the  angles  of  the  stele.  In  smaller  aerial  shoots  and 
in  the  subterranean  ones  the  central  cylinder  is  usually  protostelic. 
Sometimes  gaps  are  present  in  the  walls  of  the  tubular  cylinder  of 


I 


FIG.  179. — Diagram  of  exit  of  leaf  traces  in  the 
aerial  stem  of  Phylloglossum  (after  Bertrand). 


254 


THE  ANATOMY  OF  WOODY  PLANTS 


the  larger  stems,  but  these  in  no  case  are  related  to  outgoing  foliar 
traces.     The  general  topography  of  a  stem  of  the  type  found  in  the 

Psilotaceae  is  presented  in  Fig.  180. 

In  the  Lycopodiaceae 

in  general  the  leaf  traces 
are  ordinarily  mesarch  in 
their  organization,  a 
condition  more  or  less 
characteristic  of  the 
Lycopsida  as  a  whole. 
An  endodermis  can 
usually  be  distinguished 
about  the  foliar  strands, 
although  this  limiting 
layer  is  ordinarily  con- 
spicuous by  its  absence 
FIG.  1 80.— Transverse  section  of  the  stem  of  in  the  stem  in  most  spe- 
Psttotum.  cies  of  Lycopodium.  In 

other  representatives  of  the 
two  families  under  discus- 
sion an  external  endoder- 
mis is  usually  found  in  the 
stem,  and,  as  has  been 
shown  above,  an  internal 
endodermal  zone  is  seen  in 
the  tuberous  subterranean 
stem  of  Phylloglossum, 

In  the  genus  Selaginella 
the  fibrovascular  tissues  of 
the  axis  are  distinguished 
by  considerable  variety  in 
topography.  In  some  spe- 
cies the  stele  is  a  single 
mass,  separated  from  the 


FIG.  181. — Transverse  section  of  stem  of 
Selaginella  laevigata,  showing  a  siphonostelic 
central  cylinder. 


cortical  tissues  by  an  air-containing  region  representing  the  endo- 
dermis. In  other  species  the  fibrovascular  system  becomes  divided, 
and  the  protostelic  condition  as  a  result  passes  into  that  known  as 


THE  LYCOPODIALES 


255 


polystelic.  In  still  another  modification,  presented  by  S.  laevigata 
from  Madagascar  (Fig.  181),  a  true  siphonostele  is  exemplified  which 
is  complicated  by  the  presence  of  medullary  strands  joining  up  with 
the  walls  of  the  tube  in  the  regions  where  branches  are  given  off. 
In  the  species  under  discussion  the  traces  of  the  leaves  illustrate 
the  condition  typical  for  the  Lycopsida  and  pass  off  from  the 
cylinder  without  causing  any  x 

gaps  in  its  continuity. 

Isoetes  has  a  protostelic  stem 
which  is  remarkable  among  ex- 
isting Lycopsida  in  manifesting 
well-marked  secondary  growth 
(Fig.  182).  The  external  prod- 
uct of  cambial  activity  is  a 
radially  disposed  storage  paren- 
chyma, while  internally  the  di- 
viding layer  originates  additions 
to  the  fibrovascular  tissues 
which  are  most  commonly  inter- 
preted as  consisting  of  alternate 
zones  of  xylem  and  phloem. 
The  situation  here,  however,  is 
disputed,  and  uncertainty  per- 
sists as  a  result  of  the  indifferent 
development  of  the  tissues  re- 
sulting from  the  characteristic- 
ally aquatic  habit  of  the  plant.  The  roots  in  Isoetes  are  distin- 
guished, in  common  with  the  smaller  radical  organs  of  a  number 
of  the  lower  Lycopsida,  by  the  fact  that  they  develop  a  single 
mass  of  xylem  in  proximity  to  a  single  strand  of  phloem.  The 
leaf  is  not  worthy  of  special  note. 

In  the  Lepidodendraceae  the  stem  manifests  great  diversity 
of  structure,  as  would  naturally  be  expected  in  a  group  which 
in  Paleozoic  times  displayed  numerous  types  with  generally 
marked  secondary  growth.  The  primary  structures  of  the  stem 
were  either  protostelic  or  siphonostelic.  In  the  former  condition 
a  considerable  amount  of  parenchymatous  tissue  was  developed 


FIG.  182. — Cambial  activity  in  Isoetes 


THE  ANATOMY  OF  WOODY  PLANTS 


among  the  tracheids,  particularly  toward  the  central  region  of 
the  stele.  This  peculiar  organization  of  the  median  area  of  the 
stele  in  protostelic  lepidodendrids  is  responsible  for  a  hypothesis 
as  to  the  origin  of  the  medulla  or  pith.  Quite  generally  it  is  con- 
sidered that  by  continuing  the  process  of  transformation  of 
tracheids,  first  into  short  tracheary  elements  and  then  into  paren- 
chymatous  cells,  there  is  formed  in  the  center  of  the  stele  a  pith 

of  stelar  origin.  In 
accordance  with  this 
view  the  central  re- 
gion of  the  stele  in 
many  protostelic 
lepidodendrids  is 
called  .a  "partial 
pith."  There  is  no 
conclusive  evidence, 
however,  that  the  so- 
called  ' '  partial  pith  " 
in  reality  gives  rise 
to  the  true  medulla 
hi  those  lepidoden- 
droid  types  which 
possess  it.  More- 

FIG.  183.— Transverse  section  of  the  stem  of  Lepido-     Qver  t^e  evidence  in 


dendron  Spenceri. 


the  case  of   the 


Pteropsida,  which  are  very  much  better  displayed  in  the  period 
of  time  which  we  are  able  to  investigate,  is  distinctly  against  the 
validity  of  the  stelar  origin  of  the  pith,  since  the  medulla  in  the 
large-leaved  vascular  cryptogams  shows  very  marked  indications 
of  derivation  from  the  fundamental  system.  It  seems  on  the 
whole  more  likely  that  the  medulla  in  the  lepidodendrids,  where 
such  a  structure  is  found,  is  an  inclusion  of  fundamental  tissues 
on  the  part  of  the  stele.  This  conclusion  is  particularly  favored 
by  conditions  found  in  lepidodendroid  stems  in  which  there  is  no 
indication  of  secondary  growth,  as,  for  example,  in  Lepidodendron 
Spenceri,  shown  in  Figs.  183  and  184.  Here  the  medulla  is 
largely  occupied  by  dark-brown  sclerenchymatous  tissues  similar 


THE  LYCOPODIALES 


257 


to  those  appearing  in  the  cortical  region.  It  may  accordingly 
be  stated  that  even  the  imperfect  evidence  supplied  by  the  stem 
of  the  lepidodendrids  in  a  condition  of  obvious  degeneracy  of 
the  primary  stelar  tissues  when  they  are  first  presented  on  the  pages 
of  the  geological  record  does  not  definitely  justify  the  conclusion 
that  the  pith  is  of  stelar  origin.  In  a  later  chapter  it  will  be  made 
clear  that  the  evidence  supplied  by  the  lower  Pteropsida,  which 
is  at  once  more 
abundant  and 
more  decisive,  dis- 
tinctly vouches  for 
the  extra-stelar 
derivation  of  the 
medulla. 

The  degeneracy 
of  the  tracheary 
elements  which  has 
already  been  noted 
in  the  case  of  the 
protostelic  type  of 
cylinder  in  the 
lepidodendrids 
makes  itself  par- 
ticularly obvious 
in  the  higher 

siphonostelic  representatives  of  the  group  and  especially  in  the 
stems  included  under  the  Sigillariaceae.  The  reduction  in  amount 
of  the  primary  tissues  results  in  the  appearance  of  gaps  in  the  wall 
of  the  stelar  tube  which  are  not  related  to  appendages.  The 
central  cylinder  consequently  becomes  discontinuous,  as  has 
been  shown  in  Fig.  122,  page  170.  Since  the  later-developed 
secondary  tissues  naturally  first  appear  opposite  the  framework 
outlined  in  the  primary  wood,  the  secondary  xylem  is  likewise  at 
the  beginning  of  its  formation  discontinuous,  and  continuity 
appears  only  after  cambial  activity  has  resulted  in  the  formation 
of  a  woody  cylinder  of  some  thickness.  Where  the  primary  struc- 
ture of  the  xylem  has  undergone  the  extreme  degree  of  reduction 


FIG.  184. — Part  of  the  same,  more  highly  magnified 


258  THE  ANATOMY  OF  WOODY  PLANTS 

found  in  the  stem  of  many  of  the  Equisetales  as  well  as  of  all 
but  the  very  lowest  seed  plants,  the  initial  discontinuity  of  the 
secondary  wood  often  becomes  very  marked.  The  situation 
presented  by  these  extreme  types  unco-ordinated  with  the  eluci- 
dative anatomical  features  presented  by  more  primitive  forms  has 
been  the  cause  of  serious  misunderstanding  as  regards  the  origin 
of  the  so-called  medullary  rays.  The  case  of  the  siphonostelic 
lepidodendrids  and  Sigillariae,  as  diagrammatically  represented  in 
Fig.  1 2 2,  page  170,  seems  to  indicate  definitely  that  parenchymatous 
interruptions  in  the  secondary  cylinder  resulting  from  the  dis- 
continuity of  the  primary  wood  cannot  be  interpreted  as  true  rays 
any  more  than  are  the  gaps  related  to  the  departing  traces  of 
appendages  to  be  brought  into  the  category  of  rays.  Much 
confusion  of  definition  has  resulted  from  the  failure  to  interpret 
the  conditions  found  hi  higher  forms  in  terms  of  the  structures 
presented  in  earlier  and  more  primitive  types. 

It  has  been  made  clear  in  an  earlier  chapter  that  the  lepido- 
dendrids proper,  which  beyond  question  represent  the  more 
primitive  state  of  organization  for  the  Lepidodendraceae  as  a  whole, 
supply  evidence  for  the  derivation  of  radial  parenchyma  as  the 
result  of  the  transformation  partial  or  complete  of  radial  tra- 
cheary  strands  into  storage  parenchyma.  The  older  representa- 
tives of  the  Lepidodendraceae  show  themselves  in  this  respect  the 
most  archaic  of  all  the  vascular  plants  with  secondary  growth. 
Not  only  is  the  origin  of  radial  storage  cells  in  the  secondary  xylem 
elucidated  by  the  lepidodendrids,  but,  as  has  been  pointed  out  in 
an  earlier  chapter,  the  parenchymatous  elements  of  the  primary 
wood  have  their  origin  illustrated  in  the  conditions  found  in  the 
primary  region  of  the  stem  in  this  ancient  group  of  vascular  plants. 
It  has  been  indicated  in  Figs.  29  and  30  that  the  living  cells  occurring 
in  the  wood  of  the  primary  region  of  the  axis  in  protostelic  lepido- 
dendrids have  been  derived  by  septation  from  elements  belonging 
to  the  category  of  tracheids.  Some  of  the  resultant  elements 
persist  as  short  tracheids  with  thickly  reticulated  walls,  while 
others  maintain  a  thinner  wall  and  in  all  probability  in  life  were 
occupied  by  living  protoplasm.  In  the  siphonostelic  Lepidoden- 
draceae and  in  the  sigillarian  forms  as  a  whole  there  is  no  evi- 


THE  LYCOPODIALES  259 

dence  as  to  the  origin  of  parenchymatous  elements  in  the  primary 
xylem.  In  the  later  (Permian)  representatives  of  the  Sigillariae  the 
primary  cylinder  became  so  much  reduced  that  it  was  no  longer 
continuous.  This  topographical  condition  of  the  primary  wood 
was  responsible,  as  indicated  above,  for  a  resultant  discontinuity 
of  the  secondary  xylem.  The  processes  of  the  pith  extending 
between  the  primary  bundles  and  a  short  distance  into  the  second- 
ary cylinder  are  in  a  certain  sense  medullary  rays,  since  they  take 
their  origin  from  the  medulla;  but  they  have  nothing  in  common 
with  the  radial  masses  of  storage  tissue  resulting  from  cambial 
activity  which  characterize  the  organization  of  the  secondary 
cylinder.  Further,  they  should  not  in  any  way  be  confused  with 
foliar  gaps,  since  in  the  Lycopsida  interruptions  of  this  nature  in 
the  fibrovascular  cylinder  do  not  occur.  In  the  particular  case 
under  consideration  the  leaf  traces  originate  opposite  the  strands 
of  primary  xylem  and  do  not  subtend  the  intervals  between 
them.  It  is  clear  that  the  Lepidodendraceae,  although  entirely 
extinct,  furnish  extremely  valuable  data  for  the  elucidation  of  the 
origin  of  the  parenchyma  in  the  primary  wood  and  for  that  of  the 
radial  storage  devices  of  the  secondary  xylem.  Further,  they 
throw  a  very  clear  light  on  the  general  morphology  of  the  secondary 
woody  cylinder  in  vascular  plants,  since  the  comparative  study  of 
their  stems  from  lower  to  higher  geological  levels  makes  it  obvious 
that  the  radial  parenchymatous  bands  of  the  secondary  wood 
cannot  appropriately  be  called  medullary  rays.  They  should  be 
called  wood  rays,  as  the  inward  relation  to  the  medulla  is  neither 
a  primitive  nor  an  essential  condition. 

Not  only  in  regard  to  the  parenchymatous  structures  of  their 
primary  and  secondary  wood,  as  well  as  by  their  great  geological 
age  and  early  culmination,  do  the  lepidodendrids  in  the  large  sense 
show  themselves  to  be  primitive  representatives  of  vascular  plants, 
but  also  by  the  organization  of  the  fibrous  elements  of  the  secondary 
wood.  It  has  been  pointed  out  in  an  earlier  chapter  that  a  typical 
element  of  the  primary  wood  in  all  plants  is  the  scalariform  tracheid. 
In  the  secondary  xylem  of  plants  in  general  the  scalariform  element 
has  given  place  to  the  pitted  tracheid,  which  is  universal  for 
the  various  groups  of  gymnosperms  and  for  the  angiosperms  with 


200 


THE  ANATOMY  OF  WOODY  PLANTS 


secondary  growth.  In  the  Lepidodendraceae  in  the  narrower  sense 
the  secondary  wood  is  distinguished  from  the  primary  struc- 
tures only  by  the  presence,  exclusively,  of  radial  parenchyma 
and  by  the  radial  seriation  of  its  tracheary  elements  (Fig.  185). 
The  organization  of  the  tracheids  of  the  secondary  wood  is,  in  fact, 
identical  with  that  found  in  the  primary  region.  In  the  higher 
lepidodendroid  forms  assembled  under  the  appellation  Sigillariae 

very  frequently, 
particularly  in  the 
region  of  the 
secondary  wood 
more  remote  from 
the  pith,  the  tra- 
cheids cease  to  be 
scalariform  and 
assume  the  pitted 
type  characteristic 
of  the  gymno- 
sperms  and  other 
higher  representa- 
tives of  the  Vascu- 
lares. 

The  vascular 
strands  of  leaves  in 
the  lepidodendrids 
are  characterized, 

as  are  those  of  the  Lycopodiales  in  general,  by  mesarch  organiza- 
tion. This  condition  is  clearly  shown  in  Fig.  186.  It  is  appar- 
ent that  the  foliar  trace  is  surrounded  by  secondary  wood. 
It  has  been  stated  by  Scott  that  true  transfusion  tissue  is  present 
in  the  leaf  of  the  lepidodendrids,  but  this  statement,  in  view  of  the 
situation  present  in  the  Lycopodiales  in  general  and  in  the  Lepido- 
dendraceae in  particular,  seems  open  to  some  question,  and  certainly 
the  subject  seems  to  require  further  investigation.  The  foliar  organs 
of  the  lepidodendroid  stock  were  characterized  by  the  presence  of 
two  aerating  strands  on  either  side  of  the  foliar  trace;  these  were 
in  communication  below  with  the  external  air  through  the  agency 


FIG.  185. — Longitudinal  section  through  primary  and 
secondary  wood  of  Lepidodendron  species. 


THE  LYCOPODIALES 


261 


of  stomata,  ordinarily  accommodated  in  furrows  on  the  lower 
surface  of  the  leaf.  The  aeriferous  structures  of  the  blades  of  the 
leaves  were  continuous  with  air-containing  radial  structures  in  the 
outer  and  inner  bark  known  as  parichni.  These  are  a  noteworthy 
feature  of  structure  in  the  lepidodendrids  and  have  attracted  a 
large  amount  of  attention  from  students  of  the  group.  It  should 
be  pointed  out,  however, 
that  they  are  by  no  means 
a  unique  structure,  since 
special  aerating  devices 
are  likewise  found  present 
in  relation  to  the  leaves 
of  the  Coniferales. 

The  roots  of  the  lepido- 
dendrids have  been  the 
subject  of  much  discus- 
sion. Their  ultimate 
divisions,  the  so-called 
stigmarian  rootlets 
(Fig.  187),  are  character- 
ized by  a  very  simple 
organization,  since  only  a 
single  group  of  protoxy- 
lem  is  present.  The  root- 
lets of  this  order  divided 
dichotomously,  as  is  often 


FIG.   186. — Leaf  trace  of  a  lepidodendrid 
(after  Scott). 


the  case  with  those  of  the  living  Isoetes  and  Lycopodium.  The 
main  roots  of  the  lepidodendroid  forms  are  in  all  probability  only 
partially  known  to  us  and  present  a  curious  type  of  structure.  To 
begin  with,  there  is  a  large  pithlike  mass  around  which  is  developed 
an  extremely  small  amqunt  of  primary  wood,  at  times  so  rudimen- 
tary as  to  be  scarcely  recognizable.  The  small  degree  of  develop- 
ment of  the  primary  structures  and  the  quincuncial  arrangement  of 
the  lateral  rootlets  of  Stigmaria  have  led  to  a  great  deal  of  doubt  as 
to  their  morphological  nature.  They  have  often  been  regarded  as 
creeping  stems  or  rootstocks,  and  this  view  of  their  nature  is  found 
even  in  recent  literature  on  the  subject.  The  mass  of  anatomical 


262  THE  ANATOMY  OF  WOODY  PLANTS 

opinion  now,  however,  is  united  in  favor  of  the  view  that  they 
represent  the  larger  or  main  roots  of  lepidodendroid  forms.  They 
are,  in  fact,  to  some  extent  comparable  to  the  anomalous  roots 
found  in  the  genus  Selaginella  known  as  rhizophores.  It  seems 
highly  improbable  that  the  type  of  organization  presented  by 
Stigmaria  could  have  belonged  to  lepidodendrids  with  protostelic 
central  cylinders.  In  general,  the  subterranean  organs  of  plants 
are  less  frequently  preserved  with  structural  organization,  since 

they  are  from  the  nature 
of  things  less  likely  to 
find  their  resting-place 
in  open  bodies  of  water, 
and  this  condition  must 
usually  be  realized  in 
order  to  insure  petrifac- 
tion. It  is  accordingly 
probable  that  the  genus 
oc  Stigmaria,  as  at  present 
defined,  represents  only 
to  a  limited  extent  the 
main  radical  organs  of 
lepidodendroid  forms. 
The  Lycopodiales  as 

FIG.  187.— Rootlet  of  Stigmaria  (after  Scott)  '      r 

a  whole    are    a    group 

which  reached  its  culmination  in  the  Paleozoic  age  and  from  the 
richness  of  its  display  in  earlier  geological  times  must  be  regarded 
as  extremely  ancient.  Although  the  group  is  almost  extinct,  its 
interest  from  the  evolutionary  standpoint  is  great  by  reason  of 
its  antiquity,  which  supplies  valuable  data  for  the  elucidation 
of  some  of  the  most  important  problems  of  primitive  organiza- 
tion in  vascular  plants.  Clearly  the  group  as  a  whole  displays 
a  reduction  series  in  which  the  few  types  which  survive  under 
modern  conditions  represent,  not  primitive  states,  but  the  final 
results  of  a  process  of  simplification  extending  through  almost 
countless  ages.  Consequently  it  is  highly  inadvisable,  in  attempt- 
ing to  arrive  at  a  conception  of  the  evolutionary  significance  of 
the  group,  to  turn  exclusive  attention  to  modern  simple  forms, 


THE  LYCOPODIALES  263 

such  as  Lycopodium  or  Selaginella.  On  the  contrary,  the  most 
valuable  results  from  the  standpoint  of  the  doctrine  of  descent 
can  be  derived  from  the  study  of  the  complicated  arboreal  extinct 
types  known  as  lepidodendrids  and  sigillarians.  It  has  been  made 
clear  in  the  preceding  paragraphs  that  study  of  the  last-mentioned 
types  throws  extremely  important  light  on  the  origin  of  storage 
devices  in  the  primary  and  secondary  wood  and  makes  clear  the 
status  of  the  so-called  medullary  rays.  The  Lycopodiales,  although 
largely  extinct,  cannot  accordingly  be  neglected  by  students  of 
the  data  of  evolution,  and  they  supply  valuable  evidence,  if  any 
were  needed,  for  the  necessity  of  a  knowledge  of  extinct  forms 
as  an  indispensable  basis  for  the  understanding  of  organisms  now 
living. 


CHAPTER  XX 
THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES) 

The  forms  to  be  discussed  in  the  present  chapter  are  contrasted 
with  the  Lycopodiales  by  the  general  fact  that  the  appendages  of 
the  stem  are  arranged  in  a  whorled  or  verticillate  manner  instead 
of  in  the  spiral  fashion  characteristic  of  the  group  considered  in 
the  preceding  chapter.  Not  only,  however,  are  they  characterized 
by  the  verticillate  arrangement  of  the  appendages,  but  also  by  the 
longitudinal  ridges  and  furrows  which  mark  the  surface  of  their 
stems.  Contrary  to  the  conditions  found  in  the  furrowed  or  angular 
stems  of  certain  woody  dicotyledons,  the  traces  or  fibrovascular 
strands  of  the  leaves  correspond  in  position  to  the  salient  regions 
of  the  stem  and  not  to  its  depressions.  The  branches,  moreover, 
are  not  truly  axillary  as  is  characteristic  of  the  higher  forms,  but 
occur  at  the  node  in  alternation  with  the  foliar  organs.  This 
situation  is  very  striking  and  characteristic.  The  older  forms  in 
the  Equisetales  usually  possessed  dichotomously  divided  leaves 
or  at  least  foliar  organs  in  which  the  veins  repeatedly  forked. 
Another  important  characteristic  of  the  older  representatives  of 
the  Equisetales  was  the  continuity  of  the  ridges  and  furrows  at 
the  nodes.  In  more  modern  types  this  condition  gives  place  to 
distinct  and  finally  universal  alternation  of  the  ridges  and  furrows 
in  the  nodal  region. 

The  Equisetales  may  be  conveniently  divided  into  three  groups 
— the  Sphenophyllaceae,  the  Calamitaceae,  and  the  Equisetaceae. 
Of  these  the  first-named  may  now  be  discussed.  The  Sphenophyl- 
laceae are  Paleozoic  forms  with  slender  stems  marked  by  the  pres- 
ence of  a  relatively  small  amount  of  secondary  growth.  Their 
slender  conformation  has  led  to  the  suggestion  that  they  were 
either  vines  or  aquatics.  There  is,  however,  no  convincing  evidence 
of  the  correctness  of  either  of  these  views  in  regard  to  their  habit. 
Fig.  188  illustrates  the  organization  of  the  stem  in  Sphenophyllum 
insigne.  The  center  of  the  cylinder  is  occupied  by  the  primary 

264 


THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES)    265 


wood,  which  is  triangular  in  configuration,  with  the  small  elements 
of  the  protoxylem  at  the  angles.  The  primary  wood  presents  no 
features  of  special  interest  as  viewed  in  transverse  section  beyond 
the  fact  that  it  is  protostelic  in  its  organization  and  consequently 
lacks  a  parenchymatous  medulla  or  pith.  The  secondary  wood 
which  surrounds  the  primary  structure  is  characterized  by  the 
radial  seriation  of  its  elements  and  by  the  presence  of  medullary 
rays.  There  are  no 
other  parenchyma- 
tous structures  in  the 
secondary  wood  ex- 
cept the  rays.  The 
radial  parenchyma  of 
the  Sphenophyllaceae 
is  peculiar  in  the  fact 
that  its  cells,  instead 
of  being  strictly  elon- 
gated in  the  radial 
direction  and  at  right 
angles  to  the  longer 
axis  of  the  tracheids, 
frequently  have  their 
greatest  dimensions 
in  the  vertical  plane. 
This  situation  leads 
to  the  extension  of  the  cells  of  the  rays  along  the  edges  upward  and 
downward  among  the  tracheids  in  a  manner  simulating  true  wood 
parenchyma.  This  is,  however,  merely  an  appearance,  for  longitu- 
dinal storage  cells  of  the  type  ordinarily  known  as  wood  parenchyma 
have  not  yet  been  found  in  any  Paleozoic  wood  of  secondary  origin. 
Wood  parenchyma,  indeed,  as  has  been  indicated  in  an  earlier  chap- 
ter, was  primitively  intimately  associated  with  the  phenomenon  of 
annual  rings  which  appeared  for  the  first  time  in  the  Mesozoic  age. 
It  is  clear  from  the  description  of  the  wood  of  the  Sphenophyllaceae 
supplied  in  the  present  connection  that  it  shows,  as  indeed  might 
be  expected,  a  general  resemblance  to  that  of  the  more  ancient 
representatives  of  the  Lycopodiales.  The  tracheids  were  somewhat 


FIG.    188. — Transverse  section   of   the  stem   of 
Sphenophyllum  insigne. 


266 


THE  ANATOMY  OF  WOODY  PLANTS 


scalariform  in  their  sculpture,  but  tend,  like  those  of  the  arboreal 
Lycopodiales,  to  develop  the  pitted  condition.  The  pits,  whether 
scalariform  or  rounded,  were  found  equally  on  radial  and  tangential 
surfaces  of  the  elements  of  the  secondary  wood,  a  condition  par- 
alleled in  the  ancient  treelike  representatives  of  the  Lycopodiales. 
The  outer  region  of  the  figure  shows  the  soft  tissues  in  a  condition 
of  relative  disorganization,  which  does  not  make  their  discussion 
profitable. 

The  most  interesting  general  features  presented  by  the  anatomy 

of  the  stem  in 
Sphenophyllum  are 
its  essentially  pro- 
tostelic  character, 
the  peculiar 
organization  of  the 
rays,  and  the  tan- 
gential as  well  as 
radial  pitting  of 
the  tracheids.  The 
leaves  and  roots 
are  not  well  known 
as  to  their  ana- 
tomical organization  and  in  the  actual  state  of  our  ignorance  mani- 
fest no  features  of  unusual  interest. 

The  Calamitaceae  are  distinguished  from  the  Sphenophyllaceae 
anatomically  by  the  siphonostelic  organization  of  their  central 
cylinder.  In  the  more  ancient  types  of  Calamites  the  ridges  and 
furrows  of  the  stem  were  continuous  at  the  nodes,  precisely  as  is 
the  case  in  Sphenophyllum.  Moreover,  in  the  Archaeocalamitaceae 
the  leaves  divided  dichotomously.  In  more  modern  calamitean 
types  the  alternation  of  the  ridges  and  furrows  in  the  nodal  region 
began  to  become  a  marked  feature  of  organization  except  in  the 
cones  or  reproductive  axes,  which  adhered  to  the  more  ancient 
topography  with  non-alternation  at  the  nodes. 

Fig.  189  illustrates  the  organization  of  a  younger  stem  in  a 
calamite.  The  outer  region  of  the  axis  has  generally  disappeared 
as  a  result  of  fossilization,  but  the  woody  and  medullary  regions  are 


FIG.  189.— Transverse  section  of  a  small  stem  of 
Calamites. 


THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES)    267 

clearly  shown.  The  wood  is  apparently  entirely  secondary  in  its 
origin,  but  it  is  distinguished  by  certain  lacunae  or  cavities  which 
occupy  the  apices  of  the  wedgelike  masses  constituting  the  rather 
slender  cylinder  under  discussion.  These  cavities  indicate  the  posi- 
tion of  the  poorly  differentiated  and  evanescent  primary  wood.  So 
far  as  is  known,  the  primary  xylem  of  the  calamites  of  the  later 
Paleozoic  was  entirely  centrifugal  or  peripheral  in  its  development. 
The  great  reduction  in  the  primary  structures  of  the  calamites  has 
led  to  a  discontinuity  of  the  cylinder  comparable  to  that  found  in 
the  higher  and  more  modern  members  of  the  Sigillariae  described  in 
the  preceding  chapter.  The  lack  of  continuous  development  in  the 
cylinder  of  primary  wood  brings  with  it  a  corresponding  organiza- 
tion of  the  secondary  xylem.  The  general  result  of  the  situation 
portrayed  is  the  organization  of  the  secondary  cylinder  in  the  first 
instance  as  separate  wedges  which  finally  become  united  by  their 
increasing  breadth.  In  the  group  under  consideration  we  have 
one  of  numerous  examples  of  an  interrupted  secondary  cylinder 
resulting  from  the  meager  and  sporadic  development  of  the  primary 
wood  when  the  latter  presents  an  extreme  degree  of  reduction. 
The  pointed,  outward  excursions  of  the  pith  in  calamitean  forms 
are  in  marked  contrast  to  the  true  medullary  rays,  which  in  this 
case  are  narrow  structures.  The  rays  in  Catamites  are  characterized 
by  the  often  vertical  elongation  of  their  elements,  a  situation  which 
parallels  that  described  above  in  Sphenophyllum. 

The  slight  development  and  the  entirely  centrifugal  origin  of 
the  primary  xylem  in  the  true  calamitean  forms  is  in  marked 
contrast  to  the  conditions  presented  by  the  genus  Sphenophyllum, 
where  the  primary  structure  is  not  only  massive,  but  also  entirely 
centrad  or  centripetal  in  its  development.  There  is,  of  course, 
a  very  wide  gap  between  the  organization  of  the  axis  in  Spheno- 
phyllum and  that  found  in  Calamites.  This  gap  is  for  the  most 
part  still  unbridged  by  the  discovery  of  intermediate  forms,  but 
an  interesting  condition  is  found  in  a  stem  from  the  lower  Car- 
boniferous which  is  described  by  Scott.  In  Fig.  190  is  repro- 
duced a  somewhat  oblique  section  of  the  primary  region  of  one  of 
the  woody  wedges  of  a  calamitean  stem.  The  lacuna  or  cavity 
representing  the  position  of  the  ephemeral  primary  wood  has 


268 


THE  ANATOMY  OF  WOODY  PLANTS 


tracheary  elements,  not  only  on  the  side  which  lies  toward  the 
secondary  wood,  but  also  on  that  in  juxtaposition  to  the  medulla 

or  pith.  In  other 
words,  this  calami- 
tean  stem,  to  which 
its  discoverer  has 
applied  the  name 
Protocalamites, 
somewhat  clearly 
presents  xylem  of 
the  centrad  or  cen- 
tripetal type. 

The  secondary 
wood  of  calami tean 
forms  was  in  its 
early  organization 
largely  composed 
of  scalariform  ele- 


FIG.   190.— Transverse  section  through  the  primary 
wood  of  Protocalamites  (after  Scott). 


ments  which  in  the  ' 
later  development 
gave  place  more  or 
less  completely  to 
the  pitted  tracheids. 
Fig.  191  illustrates 
the  structure  of  the 
stem  of  a  calamite 
in  proximity  to  the 
primary  region.  It 
is  clear  that  the  tra- 
cheids are  still  very 
largely  scalariform. 
The  pitting  of  the 
tracheary  elements 
in  Catamites,  whether 
scalariform  or  rounded,  was  confined  to  the  radial  walls  of  the  ele- 
ments as  in  the  lower  gymnosperms,  and  was  not  present  on  the 


FIG.  19 1. — Longitudinal  section  of  the  wood  of  Catamites 


THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES)     269 


tangential  aspects.  True  wood  parenchyma  did  not  occur  in  the 
secondary  wood  of  Catamites.  Although  the  primary  xylem  of 
calamitean  forms  has  departed  far  from  the  primitive  condition, 
the  secondary  ligneous  organization  of  the  group  is  characterized 
by  features  which  are  only  less  primitive  than  those  presented  by 
the  lepidodendrids. 

The  organization  of  the 
leaf  in  Calamites  is  not  well 
known  as  regards  those 
features  which  are  of  inter- 
est from  the  comparative 
anatomical  standpoint,  but 
this  situation  is  fortunately 
relieved  by  certain  data 
observed  in  the  case  of  the 
foliar  organs  of  the  living 
genus  Equisetum,  which 
will  be  described  in  a  sub- 
sequent paragraph.  The 

root  of  calamitean  forms 

riG.   192. — iransverse  section  of  stem  of 
was  not  recognized  at  lirst       Equisetum  variegatum  var.  Jesupi. 

as  belonging  to  the  forms 

included  under  this  appellation  and  was  called  Astromydon.  Its 
organization  can  best  be  discussed  in  connection  with  that  of  the 
root  of  the  living  genus  Equisetum. 

In  Fig.  192  is  portrayed  the  structure  of  a  transverse  section  of 
the  stem  of  the  genus  Equisetum.  It  is  evident  that  the  center  of 
the  figure  is  occupied  by  a  large  air  space  shared  by  the  calamitean 
forms  and  indicative  both  for  the  Equisetaceae  and  their  fossil 
forebears,  the  Calamitaceae,  of  a  primitively  amphibious  habitat. 
This  central  cavity  is  often  called  the  medullary  fistula  and  in  the 
case  of  the  ancient  representatives  of  the  Equisetales  was  often 
molded  in  stone  as  pith  casts  resulting  from  mud  entering  the 
central  spaces  of  the  fallen  trunks  rotting  in  the  shallow  waters  of 
Paleozoic  lakes.  Surrounding  the  large  medullary  space  are  the 
fibro vascular  bundles,  which  are  of  small  size  and  somewhat  remote 
from  one  another.  The  strands  are  marked  by  cavities  in  their 


270 


THE  ANATOMY  OF  WOODY  PLANTS 


inner  region  which  from  their  topographical  relation  to  the  ridges 
of  the  stem  are  known  as  carinal  air  spaces.  In  alternation  with 
these  are  larger  spaces  in  the  cortex  situated  beneath  the  furrows 
of  the  stem  and  designated  consequently  as  vallecular  lacunae. 

The  cortex  is  largely  com- 
posed in  life  of  green  cells 
and  performs  the  assimila- 
tive and  transpiratory 
functions  inadequately  sub- 
served by  the  minute  leaves. 
The  fibrovascular  bundle 
must  now  receive  further 
consideration  (Fig  193). 
The  tracheary  elements  are 
scantily  present  on  the  mar- 
gins of  the  so-called  carinal 
lacunae.  This  region  is  the 
protoxylem.  Outwardly  on 
either  flank  is  seen  a  row  of 
tracheids  which  constitute 
the  metaxylem.  These  are 


FIG.  193. — Transverse  section  of  bundle  of 
rootstock  of  Equisetum  aroense. 


true  scalariform  or  reticulate  elements 
which  are  laid  down  after  the  elonga- 
tion of  the  internodes  has  come  to  an 
end.  The  two  masses  of  metaxylem 
inclose  between  them  the  tissues  of 
the  phloem,  consisting  of  larger  sieve 
tubes  and  smaller  parenchymatous 
cells.  In  Fig.  194  is  shown  a  longi- 
tudinal view  of  the  fibrovascular 
bundles  taken  a  little  to  one  side  of 
the  central  region.  To  the  left  may 
be  seen  the  carinal  cavity  containing 
remains  of  ringed  and  spiral  protoxylem.  To  the  right  appear  the 
reticulate  elements  of  the  metaxylem  which  in  the  transverse  view 
flank  the  phloem. 

The  organization  of  the  pith  and  the  distribution  of  the  endo- 
dermal  structures  in  the  genus  Equisetum  are  of  considerable 


FIG.  194. — Longitudinal  sec- 
tion of  bundle  of  rootstock  of 
Equisetum  silvaticum. 


THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES)     271 


evolutionary  interest.  In  Equisetum  arvense  (Fig.  1956),  in  all 
probability  one  of  the  most  highly  specialized  members  of  the 
group,  an  external  endodermis  alone  is  for  the  most  part  present, 

an  internal  boundary  of 
this  nature  being  found 
only  in  certain  instances 
in  the  region  of  the  nodes. 
In  E.  silvaticum,  which  may 
be  considered  a  somewhat 
less  specialized  type,  only 
an  external  endodermis  ap- 
pears in  the  aerial  stem, 
which  accordingly  presents 
a  condition  very  similar  to 
that  found  in  E.  arvense. 
In  the  subterranean  stems, 
however,  and  in  primitive 


FIG.  195  a,  b,  and  c. — Diagrams  illustrating  the  distribution  of  the  endodermis 
in  the  genus  Equisetum. 

regions  of  the  aerial  axis  an  internal  endodermis  is  likewise  seen. 
In  other  species  such  as  E.  hiemale  (Fig.  1950),  etc.,  both  internal 
and  external  limiting  layers  are  developed  throughout,  while  in 
E.  limosum  (Fig.  195^)  each  bundle  is  surrounded  by  an  individual 


272  THE  ANATOMY  OF  WOODY  PLANTS 

endodermis  except  in  the  region  of  the  nodes  where  continuous 
internal  and  external  endodermal  layers  are  seen.  The  pith  of 
the  genus  Equisetum,  particularly  in  the  region  of  the  nodes,  is 
frequently  characterized  by  the  presence  of  nests  of  dark-brown 
sclerotic  cells,  resembling  similar  structures  found  in  the  cortical 
tissues  of  both  stem  and  leaf.  On  the  grounds  of  comparative 
anatomy  we  shall  accordingly  be  compelled  to  regard  the  pith 
of  Equisetum  as  of  cortical  origin. 

The  arrangement  of  the  fibrovascular  strands  at  the  nodes  in 
living  and  fossil  representatives  of  the  Equisetales  (Equisetaceae 
and  Calamitaceae)  must  now  be  considered.  The  situation  present 
is  best  revealed  by  means  of  diagrams  (Fig.  196).  In  A  is  depicted 
the  arrangement  of  the  fibrovascular  structures  at  the  node  in  the 
vegetative  stem  of  the  living  Equisetum.  Across  the  center  of  the 
diagram  passes  a  heavy  transverse  band,  the  so-called  nodal  wood. 
In  this  the  strands  of  the  upper  and  lower  internodes  end  in  such  a 
manner  that  they  alternate  with  one  another.  The  traces  of  the 
leaves  originate  from  the  strands  of  the  lower  internode  and  thus 
subtend  the  intervals  between  the  strands  which  are  joined  with 
the  nodal  wood  from  above.  A  superficial  view  of  the  topograph- 
ical conditions  represented  here  would  result  in  the  conclusion  that 
the  Equisetaceae  are  provided  with  foliar  gaps  precisely  as  is  the 
case  in  the  Pteropsida.  A  consideration  of  B  makes  this  view  of 
the  matter  difficult  to  sustain.  In  the  figure  the  foliar  strand  is 
represented  in  radial  aspect  as  it  comes  off  from  the  fibrovascular 
tissues  of  the  axis.  It  is  clear  that  the  trace  of  the  leaf  takes  its 
origin  below  the  so-called  nodal  wood  and  passes  out  without 
showing  any  foliar  gap  above  it.  It  is  true  that,  as  is  indicated  in 
both  A  and  B,  a  gap  is  present  subtending  the  foliar  trace  above 
the  continuous  zone  of  the  wood  at  the  node;  but  a  consideration 
of  the  historical  and  comparative  anatomical  data  makes  it  difficult 
indeed  to  regard  the  gap  in  question  as  a  foliar  one.  In  C  is  shown 
the  arrangement  of  the  strands  of  the  primary  wood  in  an  ancient 
calami tean  form  (Archaeocalamites) .  It  is  evident  in  this  case 
that  the  strands  of  the  upper  and  lower  internodes,  instead  of 
meeting  the  nodal  wood  in  alternation  as  they  typically  do  in 
Equisetum,  exactly  coincide  with  one  another  and  are  not  subject 


THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES)     273 

to  alternation.  As  a  result  of  this  condition  the  leaf  traces,  which 
here,  as  in  the  modern  type,  originate  from  the  lower  internodal 
strand,  subtend,  not  a  gap  above  the  nodal  wood,  but  the  strand 
of  the  upper  internode.  It  is  thus  clear  that  the  older  condition 


FIG.  196  a,  b,  c,  and  d. — Diagrams  illustrating  the  relations  of  the  bundles  in 
Equisetum  and  Archeocalamites.     Explanation  in  the  text. 

for  the  Equisetales  is  one  in  which  there  cannot  possibly  be  a  gap 
corresponding  to  a  leaf,  even  above  the  so-called  nodal  wood.  A 
radial  view  of  the  situation  in  Archaeocalamites  makes  the  topog- 
raphy still  more  clear,  and  this  is  furnished  in  D.  Obviously  on 
historical  grounds  the  Equisetales  are  without  foliar  gaps  and 


274 


THE  ANATOMY  OF  WOODY  PLANTS 


hence  are  to  be  regarded  as  Lycopsida.  The  comparative  anatom- 
ical evidence  on  this  point  is  equally  unequivocal.  In  the  repro- 
ductive axes  or  cones  of  both  Calamitaceae  and  Equisetaceae  the 
strands  typically  fail  to  alternate  at  the  nodes,  and  the  traces  of 
the  sporophylls  are  consequently  quite  without  corresponding  gaps. 
There  apparently  can  be  no  question  on  anatomical  grounds  that 
the  Equise tales  are  justly  included  in  the  Lycopsida. 

If  the  evidence  as  to  the 
relationship  of  the  Equisetales  to 
the  phylum  Lycopsida  is  clear  on 
anatomical  grounds,  it  is  equally 
definite  from  a  consideration  of 
the  features  of  organization^  the 


FIG.  197. — Longitudinal  section  of  bundle  in  vegetative  and  reproductive  leaves 
of  Equisetum  (after  Eames) . 

cones.  In  the  Sphenophyllaceae  and  Calamitaceae  the  sporangia 
or  sporangiophores  are  known  to  be  ventral  appendages  of  the 
sporophylls  and  thus  present  the  condition  characteristic  of  the 
Lycopsida.  On  the  basis  of  both  reproductive  and  anatomical 
features  the  group  under  discussion  clearly  belongs  under  the  large 
heading  of  Lycopsida. 

The  leaf  in  the  genus  Equisetum  is  of  considerable  interest  in 
view  of  the  fact  that  it  displays  the  presence  of  centripetal  or 
cryptogamic  xylem  which  has  entirely  disappeared  in  the  stem. 
Fig.  1970  illustrates  the  organization  of  the  trace  of  the  vegetative 
leaf  of  E.  maximum.  It  is  obvious  that  the  xylem  includes  a  central 


THE  EQUISETALES  (INCLUDING  SPHENOPHYLLALES)     275 


spiral  protoxylem  region  flanked  inwardly  and  outwardly  by 
reticulate  metaxylem.  The  condition  present  is,  in  fact,  mesarch 
and  strikingly  resembles  the  common  anatomical  situation  in  the 
leaf  of  the  Lycopodiales  as  described  in  an  earlier  chapter.  In 
the  sporophyll  of  the  living  representatives  of  the  Equisetales  the 
mesarch  structure  of  the  trace  is  even  more  conspicuous  than  in 
the  vegetative  leaf.  The  situation  in  this  respect  is  shown  for 
E.  palustre  (b}.  But  the  reproductive  leaf  not  only  manifests  cen- 
tripetal wood  in  its  trace,  but 
it  also  presents  an  equally 
significant  condition  in  the  rela- 
tion of  the  phloem  to  the  xylem. 
In  c  is  shown  a  transverse  sec- 
tion of  the  trace  of  the  sporo- 
phyll in  'E.  hi  em  ale.  Sieve 
tubes  can  plainly  be  seen  sur- 
rounding the  xylem  elements, 
while  the  latter  have  in  their 
midst  a  more  or  less  obvious 
lacuna  representing  the  evanes- 
cent protoxylem.  On  the 
grounds  of  comparative  anat- 
omy it  is  clear  that  the 

FIG.  198. — Transverse  section  of  the 

Equisetaceae  once  possessed     root  of  Equisetum  hiemale. 
centripetal  wood  in  the  stem. 

That  this  was  the  former  situation  for  the  stock  as  a  whole  is  clearly 
indicated  by  the  anatomy  of  Protocalamites  shown  in  Fig.  190.  It 
is  further  rendered  highly  probable  by  the  concentric  as  well  as  the 
mesarch  organization  of  the  trace  of  the  sporophyll  in  the  living 
genus  that  the  bundles  of  the  axis  were  formerly  concentric  in  their 
organization.  This  condition  must,  however,  have  been  realized  in 
the  extremely  remote  past,  as  no  indication  of  concentric  structure 
has  been  found  in  the  stems  of  even  the  most  ancient  anatomically 
investigated  remains  of  calamitean  forms. 

The  root  in  the  genus  Equisetum  (Fig.  198)  shows  the  presence 
of  four  protoxylem  groups  alternating  with  as  many  clusters  of 
elements  of  the  phloem.  The  metaxylem  consists  ordinarily  of  a 


276  THE  ANATOMY  OF  WOODY  PLANTS 

single  large  central  tracheid  in  which  the  four  masses  of  protoxylem 
unite.  In  the  smaller  roots  of  Catamites  the  same  general  organi- 
zation is  found  as  presents  itself  in  Equisetum.  The  radical  organs 
of  this  extinct  group  of  the  Equise tales  are  known  as  Astromyelon, 
a  name  given  before  their  connection  with  calamitean  stems  was 
known. 

In  the  Equisetales  as  a  whole,  represented  by  the  Sphenophyl- 
laceae,  Calamitaceae,  and  Equisetaceae,  very  marked  features  of 
organization  are  present.  In  fact,  the  group  shows  characteristics 
which  may  well  be  denominated  unique.  It  is  clear  that  the  group 
is  of  very  ancient  origin,  since  when  it  first  comes  into  view  it  is 
distinctly  set  off  from  the  other  large  alliance  of  the  Lycopsida,  the 
Lycopodiales,  by  the  whorled  character  of  its  appendages,  the 
ridges  and  furrows  of  the  .stem,  and  the  sporangiophoric  manner  of 
reproduction.  Scott  has  regarded  these  features  as  sufficiently 
distinctive  to  warrant  the  establishment  of  a  third  great  phylum 
of  vascular  plants,  the  Sphenopsida.  This  group  he  considers  as 
on  the  whole  more  nearly  allied  to  the  Pteropsida  than  to  the  Lycop- 
sida. In  view  of  the  absence  of  foliar  gaps  in  the  series  under 
discussion  in  the  present  chapter,  particularly  clear  when  both  fossil 
and  living  forms  are  brought  into  consideration,  there  does  not 
seem  to  be  any  adequate  anatomical  evidence  to  support  the  sepa- 
ration of  the  Equisetales  under  the  heading  of  Sphenopsida.  The 
reproductive  characters  of  the  equisetal  series  are  likewise  most 
easily  reconciled  with  an  affinity  to  the  Lycopsida  in  general  and 
to  the  Lycopodiales  in  particular.  Scott  regards  the  Psilotaceae  as 
more  nearly  related  to  the  Sphenophyllaceae  than  to  the  lycopo- 
dineous  forms.  The  evidence  in  favor  of  this  view  does  not  seem, 
however,  to  be  of  a  compelling  character. 


CHAPTER  XXI 
THE  FILICALES 

This  group  of  vascular  plants  presents  the  features  of  the 
Pteropsida  in  their  most  primitive  and  least  modified  condition. 
Large  leaves  are  consequently  the  rule,  and  these  normally,  when 
functioning  as  sporophylls,  bear  numerous  sporangia  on  the  lower 
or  abaxial  (dorsal)  surface.  When  the  central  cylinder  of  the 
stem  is  siphonostelic,  as  is  most  frequently  the  case,  the  traces  of 
the  leaves  take  their  departure  from  the  wall  of  the  stelar  tube 
with  the  formation  of  foliar  gaps  subtending  the  departing  strands. 
In  the  Filicales  reproduction  is  always  by  means  of  spores,  which 
are  in  general  isosporous,  but  which  in  a  few  instances  represent  the 
heterosporous  condition.  The  Filicales  constitute  a  remarkably 
clearly  denned  group,  at  least  so  far  as  their  modern  representatives 
are  concerned;  and  the  only  family  which  has  had  its  affinities 
with  the  Pteropsida  brought  into  question  is  the  Ophioglossaceae, 
regarded  in  some  quarters  as  derived  from  lycopsid  ancestry. 
This  attribution  of  affinity,  however,  is  not  now  considered  justi- 
fied. The  Filicales  constitute  the  largest  element  composed  of 
vascular  cryptogams  in  the  existing  flora  of  our  earth  and  are 
on  that  account  of  great  importance  from  the  evolutionary  stand- 
point. Anatomical  problems  which,  in  the  Lycopsida,  are  difficult 
of  elucidation  by  reason  of  the  large  degree  to  which  the  group  has 
suffered  extinction  in  the  existing  flora  are  much  more  advanta- 
geously approached,  in  the  group  under  consideration,  as  a  conse- 
quence of  the  large  number  of  forms  which  are  offered  for  study 
by  the  existing  pknt  population  of  the  earth.  The  value  of 
the  filicinean  Pteropsida  is  particularly  great  in  respect  to  the 
fibrovascular  structures,  and  an  attempt  will  be  made  in  the 
present  connection  to  utilize  these  to  the  full.  Naturally,  in  a 
group  surviving  in  relatively  large  numbers  only  the  more  salient 
and  significant  facts  can  be  brought  into  prominence  in  an 
elementary  treatise  like  this. 

277 


278 


THE  ANATOMY  OF  WOODY  PLANTS 


The  stem  as  the  most  plastic  of  the  organs  in  vascular  plants 
presents  the  greatest  variety  of  structure  in  the  Filicales.  The 
leaf,  and  particularly  the  root,  offer  little  diversity  of  organization 
and  may  consequently  be  dismissed  with  relatively  slight  con- 
sideration. Anatomically  the  stem  presents  itself  in  the  case 
of  the  Filicales  under  two  main  conditions:  the  protostelic,  in 
which  there  is  no  medulla  present  in  the  fibrovascular  system, 

and  the  siphonostelic, 
characterized  by  the 
existence  of  a  central 
mass  of  parenchyma 
known  as  the  medulla 
or  pith.  The  first  con- 
dition is  represented 
in  Fig.  199,  portray- 
ing the  transverse 
section  of  the  stem  of 
a  species  of  Glei- 
chenia.  The  siphon- 
ostelic modification  is 
delineated  in  Fig.  200, 
reproducing  the 
transverse  aspect  of 
the  stem  of  the 
maidenhair  fern, 
Adiantum  pedatum.  In  the  second  figure  we  find  the  fibrovascular 
tissues  organized  in  the  form  of  a  tube,  limited  both  internally  and 
externally  by  an  endodermal  boundary  which  becomes  continuous 
around  the  margins  of  the  gaps  caused  by  the  exit  of  the  traces  of  the 
lateral  branches  and  leaves.  Not  only  is  the  tubular  central  cylin- 
der bounded  continuously  by  an  endodermal  layer,  but  it  is  likewise 
characterized  in  the  particular  case  under  discussion  by  an  inner 
and  outer  lining  of  phloem.  In  the  walls  of  the  stelar  tube  so 
organized  there  are  gaps  formed  in  connection  with  the  exit  of 
the  fibrovascular  strands  leading  to  both  leaves  and  lateral  branches. 
In  addition  to  these  there  may  be  interruptions  in  the  continuity 
of  the  fibrovascular  hollow  cylinder  which  are  not  related  to 


FIG.   199.— Transverse  section  of   the  stem  of 
Gleichenia  species. 


THE  FILICALES  279 

departing  strands  of  any  organs.  The  root  is  not  responsible  for 
the  appearance  of  a  gap  in  the  wall  of  the  stelar  tube  unless  it 
happens,  as  is  sometimes  the  case,  to  be  closely  related  to  a  foliar 
organ.  Under  these  conditions  the  apparent  gap  is  naturally 
foliar  and  not  radical. 

Before  passing  to  the  discussion  of  modifications  of  the  siphon- 
ostelic  central  cylinder  as  presented  by  the  stem  in  the  Filicales 
it  will  be  well  to  consider  the  general  topography  of  the  fibrovascular 
system  in  the  various  organs.  The  account  of  the  stelar  system 


FIG.  200. — Transverse  section  of  the  stem  of  Adiantum  pedatum 

of  the  stem  outlined  in  the  preceding  paragraph  will  suffice  for 
the  cauline  organ,  so  that  it  is  possible  to  turn  at  once  to  the 
discussion  of  the  leaf  and  the  root.  As  has  been  indicated  above, 
the  foliar  trace  departs  from  the  tubular  central  cylinder  of  the 
axis  in  topographical  relation  to  a  lacuna  in  the  stelar  wall 
which  is  known  as  the  foliar  gap.  The  outgoing  foliar  trace  in 
the  lower  region  where  it  runs  in  the  stipe  or  rachis  or  even 
sometimes  in  the  subdivisions  in-  the  main  veins  of  the  flattened 
region  or  lamina,  is  concentric  or  bicollateral  in  its  organization. 
This  characterization  means  that  the  xylem  is  either  completely 
surrounded  by  phloem  or  at  least  has  phloem  on  its  two 
opposite  sides.  As  the  fibrovascular  strands  which  innervate 
the  blade  of  the  leaf  become  more  finely  divided  they  lose 


280  THE  ANATOMY  OF  WOODY  PLANTS 

their  original  concentric  or  bicollateral  structure  and  develop 
a  collateral  organization.  This  condition  of  the  finer  fibrovascular 
structure  of  the  foliar  organ  is  clearly  correlated  with  the  dorsi- 
ventral  structure  of  the  leaf  as  a  whole,  and  favors  its  function- 
ing in  relation  to  photosynthesis  and  transpiration.  The  upper 
region  of  the  foliar  strands  is  consequently  largely  under  the  sway 
of  physiological  conditions,  while  in  their  lower  course,  and  espe- 
cially before  they  have  suffered  much  in  magnitude  as  a  result  of 
subdivision,  the  ancestral  conditions  may  as  a  rule  be  more  readily 
observed.  Further,  the  more  aberrant  the  anatomical  structures 
are  in  any  given  case  the  less  will  be  the  degree  of  development 
of  the  ancestral  conditions  in  the  foliar  traces.  In  argument 
regarding  the  interpretation  of  the  fibrovascular  structures  in 
stems  the  organization  of  the  foliar  trace  after  it  has  left  the 
stele  of  the  leaf  is  of  great  importance.  In  the  root  of  such 
relatively  low  types  as  the  Filicales  little  evidence  is  supplied 
which  is  of  value  in  the  interpretation  of  the  primitive  organiza- 
tion of  the  tubular  stele  of  the  stem.  This  statement  holds, 
not  only  for  the  true  ferns,  but  also  for  those  lower  and  ancient 
gymnosperms  which  have  the  most  marked  filicinean  affinities. 
It  will  be  apparent,  however,  when  the  discussion  of  the  anatomical 
organization  of  the  axis  in  higher  gymnosperms  and  angiosperms 
is  reached,  that  the  root  assumes  an  evolutionary  significance 
which  is  not  observed  in  the  lower  groups  of  the  Vasculares.  With 
these  preliminary  remarks  it  is  possible  to  pass  with  advantage 
to  the  consideration  of  the  organization  and  evolution  of  the 
tubular  central  cylinder  of  the  axis. 

First  will  be  discussed  modifications  of  the  tubular  stele  which 
are  in  the  direction  of  greater  complexity.  In  many  ferns  the 
central  cylinder  or  stele  in  the  adult  stem  is  represented  by  a 
complex  grouping  of  strands.  In  the  bracken  fern  (Pteris  aquilina), 
as  is  shown  hi  Fig.  201,  the  older  stem  presents  in  transverse 
section  two  series  of  bundles — two  large  central  ones  and  a  ring 
of  usually  much  smaller  ones  forming  a  circle  outside  these.  The 
significance  of  the  conditions  present  in  the  stem  of  this  most 
commonly  studied  fern  have  been  very  generally  misunderstood. 
It  has  been  maintained  by  the  distinguished  French  anatomist 


THE  FILICALES  281 

Van  Tieghem  that  the  vascular  system  of  the  bracken  arises  by 
the  continued  forking  of  an  originally  simple  (protostelic)  strand. 
According  to  his  view  the  two  large  central  bundles  are  the  first 
to  appear  as  the  result  of  the  process  of  forking,  and  the  smaller 
circle  of  strands  lying  outside  these  is  formed  later.  On  account 
of  the  supposed  origin  of  the  strands  in  the  stem  of  Pteris  aquilina 
by  repeated  division,  the  name  polystelic  was  given  by  Van  Tieghem 
to  this  and  similar 
conditions  of  ana- 
tomical organization 
in  the  ferns  and  their 
allies.  As  a  matter 
of  fact  the  situation 
is  very  different 
indeed  from  that 
indicated  by  the 
term  polystelic.  In 
Fig.  2O2a  is  shown 
the  transverse  sec- 
tion of  a  very  young 
stem  of  the  bracken 
when  it  is  still  in  the  „ 

.tic.  201. — Complicated  bundle  system  of  Pteris  aqmhna 

upright   condition 

and  has  not  given  rise  to  the  subterranean  horizontal  branches 
which  come  into  existence  at  a  comparatively  early  stage.  The 
stem  at  this  age  is  obviously  siphonostelic  and  is  marked  by  a  gap 
correlated  to  an  outgoing  leaf  trace.  In  &  a  later  stage  of 
development,  characteristic  of  the  young  horizontal  stem,  is 
shown.  Here  the  tubular  central  cylinder  is  giving  off  internally 
and  toward  the  center  a  medullary  strand.  Later  the  single 
medullary  strand  becomes  double  and  the  condition  attained  in 
the  adult  is  reached.  It  is  clear  from  an  examination  of  the 
actual  course  of  development  in  the  stem  of  the  bracken  fern 
that  the  medullary  strands  are  not  formed  first,  as  is  assumed 
by  Van  Tieghem,  nor  does  the  stelar  system  of  the  adult  result 
from  repeated  bifurcation  of  an  originally  single  strand.  On  the 
contrary,  the  course  of  development  presents  first  a  protostelic 


282 


THE  ANATOMY  OF  WOODY  PLANTS 


condition  in  which  no  medullary  tissues  are  present,  a  phase 
followed  by  the  siphonostelic  one,  which  in  turn  develops  medullary 
strands  from  the  inner  surface  of  its  walls.  The  outer  series  of 
bundles  is  consequently  antecedent  to  the  medullary  bundles,  a 
situation  which  may  be  readily  inferred  without  the  study  of 
the  young  plant  from  the  fact  that  the  roots  are  attached  to  the 
external  strands.  Not  only  does  the  bracken  manifest  the  general 


FIG.  202. — Diagram  to  illustrate  the  development  of  the  stem  in  Pteris  aquilina. 
Explanation  in  the  text. 

conditions  described  above,  but  all  ferns  with  complicated  arrange- 
ments of  conducting  strands  in  the  stem  can  readily  be  included 
under  the  same  general  statement.  This,  for  example,  is  true 
of  the  complex  stem  of  the  Cyathaceae  or  tree-ferns  and  also  of 
that  of  the  large  tropical  ferns  known  as  the  Marattiaceae.  It  is 
apparent  from  the  account  supplied  in  the  present  connection 
that  the  tubular  condition  is  both  typical  and  primitive  for  the 
ferns  in  general,  with  the  sole  exception  of  those  forms  in  which 
the  organization  of  the  stele  maintains  the  original  protostelic 
condition.  This  type  of  structure  is,  however,  comparatively 
rare  in  existing  ferns,  although  it  has  been  found,  as  would 


THE  FILICALES  283 

naturally  be  expected,  much  more  generally  in  Paleozoic  types 
belonging  to  the  Filicales. 

Before  proceeding  further  with  the  question  of  the  organization 
of  the  tubular  or  siphonostelic  type  of  central  cylinder,  it  will  be 
well  to  discuss  the  important  problem  of  the  origin  of  the  pith 
or  medulla  in  the  Filicales.  The  situation  in  this  group  is  much 
more  favorable  than  it  is  in  the  Lycopsida,  living  and  extinct, 
because  the  facts  at  our  disposal  are  much  fuller  and  as  a  conse- 
quence put  the  whole  subject  on  a  more  satisfactory  footing. 
There  are  two  views  in  regard  to  the  origin  of  the  medulla  in 
stems  of  the  type  here  designated  siphonostelic.  One  hypothesis 
considers  it  as  originating  from  the  central  region  of  the  xylem 
of  the  stele  by  the  transformation  of  tracheary  elements  into 
parenchyma.  It  has  been  shown  in  earlier  chapters  that  there 
can  be  no  question  that  parenchymatous  elements  may  be  derived 
from  tracheids  in  the  primary  wood,  and  the  evidence  in  this 
direction  clearly  points  to  a  possible  origin  of  the  medullary  tissues 
from  transformed  tracheids.  Indeed,  in  the  case  of  the  lepidoden- 
drids  the  facts  which  favor  the  tracheary  origin  of  the  medulla 
are  not  without  importance.  Therefore,  if  we  are  to  regard  the 
medulla  as  derived  from  tracheary  tissue  on  the  basis  of  the  exist- 
ence of  transitional  stages  between  tracheids  and  parenchyma, 
it  will  be  only  fair  on  the  other  side  to  assume  that  where  elements 
clearly  of  a  cortical  nature  occur  in  the  region  of  the  pith  they 
afford  evidence  for  the  other  hypothesis  of  the  origin  of  medullary 
parenchyma,  namely,  its  derivation  from  the  included  cortex. 
It  is,  of  course,  illogical  to  admit  evidence  for  a  derivation  of 
medullary  structures  from  tracheids  and  at  the  same  time  to 
reject  equally  cogent  data  as  to  the  origin  of  the  pith  from  cortical 
tissues.  Even  in  the  lepidodendrids  the  evidence  for  the  trache- 
ary derivation  of  the  medullary  structures  is  not  complete, 
for  transitions  from  tracheids  to  parenchymatous  cells  are  pre- 
sented only  by  protostelic  stems  and  are  entirely  absent  in 
tubular  cylinders.  Indeed,  in  cases  where  secondary  growth 
is  not  characteristic  of  the  siphonostele  in  this  group,  the  medullary 
region  of  the  stele  is  frequently  occupied  by  sclerenchymatous 
elements  similar  to  those  which  occur  in  the  cortex.  The  hypothesis 


284  THE  ANATOMY  OF  WOODY  PLANTS 

of  the  tracheary  origin  of  the  pith  is  also  open  to  question  in  other 
representatives  of  the  Lycopsida.  In  Phylloglossum  the  medulla 
in  the  region  of  the  tuberous  portion  of  the  stem  is  actually  sur- 
rounded by  an  endodermal  layer,  a  structure  peculiar  to  the  cortex. 
In  Selaginella  laevigata,  also,  the  tubular  stele  is  bounded  internally 
as  well  as  externally  by  a  well-defined  endodermal  layer,  and  the 
tissues  of  the  pith,  further,  clearly  resemble  those  of  the  cortex. 
In  both  Psilotum  and  Tmesipteris  the  medullary  tissues  are  fre- 
quently composed  to  a  large  extent  of  brown  sclerenchymatous 
elements  resembling  similar  structures  found  in  the  external 
fundamental  tissues.  In  the  genus  Equisetum  dark-brown  scle- 
renchyma  is  present  in  the  medulla  and  the  cortex.  It  is  clear 
on  the  basis  of  data  derived  from  resemblances  between  the  tissues 
of  the  pith  and  the  cortex,  on  the  one  hand,  and  the  tracheary 
tissue,  on  the  other,  that  the  preponderance  of  evidence  weighs 
heavily  on  the  side  of  the  fundamental  or  cortical  origin  of  the 
medulla.  And  heavily  as  the  scale  seems  to  incline  on  the  side 
of  the  fundamental  derivation  of  the  pith  in  the  Lycopsida,  it 
seems  entirely  overwhelming  in  the  case  of  the  Filicales. 

There  are  two  general  arguments  which  have  been  invoked 
against  the  origin  of  the  pith  from  the  fundamental  tissues  in 
the  Filicales.  One  is  the  denial  that  it  is  possible  for  the  stele 
to  include  the  tissues  of  the  cortex.  This  reasoning  cannot  be 
given  very  serious  weight  in  view  of  the  fact  that  in  certain  fili- 
cinean  steles — for  example,  that  of  the  polypodiaceous  genus 
Onoclea  and  the  schizeaceous  genus  Anemia — the  possibility  of 
the  inclusion  of  fundamental  tissues  within  the  tubular  stele  must 
apparently  be  granted,  because  the  epidermis  and  chaffy  ramentum 
as  well  as  the  outside  air  are  actually  included  within  the  medullary 
region  of  the  central  cylinder.  Moreover,  the  phloem,  a  tissue 
primitively  occurring  on  the  outer  surface  of  the  central  cylinder, 
is  frequently  included  in  its  interior.  If  any  further  evidence 
were  needed  as  to  the  possibility  of  the  xylem  including  tissues 
of  another  morphological  category,  it  is  furnished  by  the  case 
of  certain  filicinean  foliar  traces  within  which  it  is  universally 
admitted,  even  by  the  most  convinced  adherents  of  the  hypothesis 
of  the  tracheary  origin  of  the  pith,  fundamental  tissues  may  not 


THE  FILICALES  285' 

only  be  included,  but  in  some  instances  may  be  entirely  shut  off 
from  the  similar  tissues  outside.  Gr>od  illustrations  of  this  con- 
dition are  provided  by  species  of  the  Gleicheniaceae,  Polypodiaceae 
(notably  the  common  bracken  fern),  and  Marattiaceae.  The 
possibility,  then,  of  the  inclusion  of  the  pith  within  the  stele  must 
apparently  be  granted. 

The  second  argument  against  the  cortical  origin  of  the  medulla 
is  directed  against  the  evidence  based  on  the  similar  histological 
nature  of  pith  and  cortex  in  many  cases,  a  resemblance  which 
is  the  more  marked  the  more  the  medullary  tissues  are  in  histo- 
logical continuity  with  those  lying  outside  the  tubular  stele.  It 
is  assumed  by  the  advocates  of  the  tracheary  origin  of  the  pith 
that  the  striking  histological  similarity  which  often  exists 
between  the  medulla  and  the  cortex  is  merely  "physiological." 
This  argument  would  be  worth  more  if  its  exponents  did  not  admit 
all  histological  evidence  in  favor  of  the  tracheary  origin  of  the 
medullary  tissues  while  denying  that  indicating  their  derivation 
from  the  fundamental  system.  Such  argument  is  clearly  fallacious, 
and  the  logical  procedure  is  to  admit  to  equal  consideration  evi- 
dence for  the  tracheary  hypothesis  of  the  appearance  of  the  medulla 
on  the  one  hand  and  that  for  its  derivation  by  the  inclusion  of 
cortical  or  fundamental  tissues  on  the  other.  The  advocates  of 
the  cortical  origin  of  the  central  parenchyma  of  the  tubular  stele 
are  apparently  advantageously  situated  in  this  respect,  for  they 
can  equally  well  emphasize  the  extremely  abundant  data  for 
the  origin  of  the  pith  from  the  fundamental  system  and  at  the 
same  time  explain  away  the  meager  evidence  for  the  tracheary 
derivation  of  the  medullary  tissues.  So  far  as  the  plentiful  evi- 
dence in  the  case  of  the  Filicales  is  concerned,  it  seems  beyond 
reasonable  doubt  that  the  median  parenchyma  of  the  tubular 
or  siphonostelic  central  cylinder  has  come  from  the  outside  and 
is  not  the  result  of  internal  differentiation  within  the  stele. 

The  question  of  the  origin  of  the  pith  is  a  very  important  one 
from  the  phylogenetic  standpoint,  since  correct  views  in  regard 
to  this  matter  are  necessary  for  the  interpretation  of  the  evolution 
of  the  tubular  cylinder  from  lower  to  higher  forms.  It  would 
be  going  beyond  the  range  of  the  present  somewhat  elementary 


286  THE  ANATOMY  OF  WOODY  PLANTS 

treatise  upon  the  anatomy  of  the  vascular  plants  to  elaborate  at 
any  length  the  question  of  the  phylogeny  of  the  tubular  central 
cylinder,  important  as  this  is  for  the  proper  understanding  of 
the  higher  plants  in  the  light  of  the  doctrine  of  descent.  The 
whole  situation  may  conveniently  be  illustrated  by  reference  to 
the  Osmundaceae.  In  discussing  the  evolutionary  principles 
involved  in  the  development  of  the  siphonostele  in  this  group,  we 


FIG.  203. — Transverse  section  of  the  stem  of  Osmundites  skidegatensis 

shall  appropriately  start  with  types  from  earlier  geological  periods. 
In  Fig.  203  is  shown  a  photograph  of  the  central  cylinder  of  a 
fossil  osmundaceous  stem  from  the  Lower  Cretaceous  of  Western 
Canada.  In  the  stem  under  discussion  the  magnitude  of  the 
stelar  system  is  much  greater  than  that  found  in  any  living  species 
belonging  to  the  tribe  and,  in  fact,  more  clearly  resembles  the 
conditions  presented  in  a  large  stem  of  a  fern  of  the  ordinary  type. 
The  pith  as  well  as  the  cortex  is  occupied  by  numerous  dark  masses 
of  brown  sclerenchyma — a  condition  closely  resembling  the  state 
found  in  siphonostelic  stems  of  ordinary  ferns.  An  inspection 
of  the  periphery  of  the  stele  shows  the  presence  of  numerous 


THE  FILICALES 


287 


foliar  gaps,  through  some  of  which  the  cortex  and  the  pith  are 
actually  continuous,  as  is  made  clear  by  Fig.  204,  representing 
a  small  segment  of  the  fibrovascular  cylinder.  In  Fig.  205  is 
shown  a  still  more  magnified  view  of  the  marginal  region  of  the 
stele,  and  it  is  here  apparent  that  phloem  is  present  both  on  the 
inside  and  on  the  outside  of  the  stelar  tube.  The  situation  re- 
vealed by  the  three  figures  of  the  stem  of  Osmundites  skidegatensis 
shown  above 
makes  it  evident 
that  in  this  ancient 
representative  of 
the  group  a  condi- 
tion of  organiza- 
tion more  nearly 
resembling  that 
found  in  our  ordi- 
nary polypodiace- 
ous  ferns  was 
present.  The  only 
important  differ- 
ence is  furnished 
by  the  extremely 
numerous  foliar 
gaps  correspond- 
ing to  the  more 
crowded  condition 

of  the  leaves.     This,  of  course,  is  not  a  morphologically  important 
distinction. 

After  the  description  of  an  osmundaceous  fern  from  the  Meso- 
zoic,  the  stele  of  living  species  of  the  Osmundaceae  may  con- 
veniently be  considered.  Fig.  206  illustrates  the  organization 
of  the  central  cylinder  of  Osmunda  cinnamomea,  the  cinnamon 
fern.  The  fibrovascular  tissues  in  this  case  are  characterized  by 
the  same  numerous  foliar  gaps  as  are  found  in  0.  skidegatensis, 
but  the  cortex  and  the  pith  do  not  communicate  through  them 
as  in  the  Mesozoic  type.  Further,  the  phloem  in  the  specimen 
under  discussion,  unlike  that  present  in  the  cretaceous  species, 


FIG.  204.— Part  of  the  last,  more  highly  magnified  tc 
show  the  leaf  gaps. 


288 


THE  ANATOMY  OF  WOODY  PLANTS 


is  confined  to  the  outside  of  the  xylem.  Endodermal  layers, 
both  internal  and  external,  are  present,  and  the  pith  often  contains 
patches  of  brown  sclerotic  cells  similar  to  those  found  in  the  cortex. 
In  spite  of  the  fact  that  the  bundles  of  the  central  cylinder  of  the 
stem  are  collateral,  those  of  the  leaves  are  concentric  in  organiza- 
tion. Applying  the  principles  of  comparative  anatomy  to  the 
situation,  we  find  that  in  O.  cinnamomea  the  pith  and  cortex 

are  of  common 
origin  and  that  the 
bundles  of  the 
stele,  on  the  evi- 
dence furnished  by 
the  foliar  strands, 
were  formerly  con- 
centric in  structure. 
The  conclusion 
which  is  reached 
from  the  considera- 
tion of  the  stem  of 
the  species  under 
discussion,  in  the 
light  of  universally 
valid  principles  of 
comparative  anat- 
omy, is  that  its 
tubular  stele  was 
formerly  concentric  and  that  the  foliar  gaps  were  once  large 
enough  to  permit  of  the  joining  of  cortical  and  medullary  tis- 
sues with  one  another.  The  inferences  drawn  from  the  data 
indicated  above  are,  moreover,  entirely  justified  by  the  considera- 
tion of  the  anatomy  of  the  Lower  Cretaceous  species  described 
in  the  preceding  paragraph.  In  particularly  vigorous  specimens 
of  the  stem  in  O.  cinnamomea  the  ancestral  condition,  moreover, 
frequently  returns,  for  both  open  foliar  gaps  and  internal  phloem 
are  often  seen  in  such  axes.  In  O.  regalis  among  living  repre- 
sentatives of  the  Osmundaceae  medullary  brown  sclerenchyma 
is  occasionally  found  in  the  pith,  although  an  internal  endo- 


FiG.  205. — Part  of  the  stem  of  O.  skidegatensis,  still 
more  highly  magnified  to  show  the  presence  of  both 
internal  and  external  phloem. 


THE  FILICALES  289 

dermis  has  never  been  observed.  In  0.  claytoniana  the  pith 
never  resembles  the  cortical  tissues,  and  neither  internal  phloem 
nor  endodermis  is  known  to  exist  even  in  the  most  vigorous  speci- 
mens. In  the  three  living  species  cited  in  the  present  chapter 
we  find  a  reduction  series  in  which  O.  cinnamomea  represents  the 
most  primitive  condition  and  O.  claytoniana  the  most  aberrant. 

A  quite  different  interpretation  is  put  on  the  anatomical  facts 
cited  in  the  two  preceding  paragraphs  by  those  who  adhere  to 


FIG.  206. — Photographs  of  the  fibrovascular  region  of  the  stem  in  Osmunda 
cinnamomea,  showing  internal  endodermis  and  medullary  sclerenchyma. 

the  hypothesis  of  the  tracheary  origin  of  the  pith.  To  those  who 
adopt  the  view  that  the  medulla  is  derived  from  the  substance  of 
the  stele,  the  type  of  Osmunda  without  either  internal  phloem  or 
endodermis  is  the  more  primitive,  while  that  in  which  both  these 
structures  occur  is  more  modern.  Those  who  regard  the  anatomi- 
cal facts  from  this  standpoint  are  in  a  position  that  offers  many 
difficulties.  First  of  all,  they  have  to  assume  that  an  old  type 
like  Osmunda  skidegatensis  has  gained  a  high  degree  of  development 
and  that  the  modern  forms  without  open  foliar  gaps,  sclerotic 
pith,  internal  endodermis,  and  internal  phloem  represent  in  reality 
a  more  primitive  condition  of  organization  than  does  the  Mesozoic 
type  cited  above.  This  view  is  not  only  not  in  harmony  with 


290  THE  ANATOMY  OF  WOODY  PLANTS 

the  paleontological  facts,  but  is  likewise  at  variance  with  the 
fundamental  principles  of  comparative  anatomy.  It  presents 
the  further  serious  difficulty  of  supposing  that  the  simpler  type 
of  stele  characteristic  of  the  more  modern  representatives  of  the 
Osmundaceae,  although  primitive,  has  nevertheless  come  from  a 
more  complicated  condition  in  the  past.  Finally,  the  hypothesis 
of  the  greater  primitiveness  of  tubular  steles  without  internal 
phloem  supplies  no  valid  explanation  of  the  frequent  occurrence 
of  internal  phloem,  internal  endodermis,  and  cortical  sclerenchyma 
in  the  medullary  region  of  various  existing  species  of  the  Osmun- 
daceae. The  view  that  such  structures  where  they  occur  are 
"physiological"  has  little  to  commend  it  and  is,  moreover,  a 
hypothesis  which  cuts  both  ways;  for  if  we  are  to  interpret  cortical 
structures  occurring  in  the  medulla  as  merely  physiological,  we 
must  likewise  consider  the  possibility  of  a  similar  explanation 
of  the  rare  and  quite  exceptional  occurrence  of  a  so-called  "mixed 
pith,"  consisting  partially  of  tracheary  tissue  and  partially  of 
parenchyma. 

The  Osmundaceae  on  the  whole  present  the  evidence  in  regard 
to  the  evolution  of  the  tubular  stele  in  later  geological  times  more 
clearly  than  does  any  other  group  of  ferns.  It  should,  however, 
be  emphasized  that  in  practically  all  cases  where  the  siphonostelic 
central  cylinder  without  internal  phloem  or  endodermis  is  present 
there  are  found  elements  in  the  region  of  the  medulla  which 
present  the  characteristics  of  cortical  tissues.  It  seems  illogical 
to  interpret  these  structures  as  possessing  in  every  case  merely  a 
physiological  significance,  particularly  since,  in  view  of  their 
imperfect  and  sporadic  development,  it  is  difficult  to  attribute 
to  them  any  functional  importance  whatever.  The  structures 
in  question  possess,  in  fact,  all  the  criteria  of  vestigial  features 
persisting  from  an  earlier  more  complicated  condition  of  organiza- 
tion. Our  knowledge  of  groups  which  have  their  climax  of  devel- 
opment in  the  past  justifies  the  view  that  evolution  in  decadent 
series  proceeds  by  simplification.  A  final  general  objection  to 
the  tracheary  hypothesis  of  the  origin  of  the  pith  is  the  outstanding 
fact  that  the  more  modern  groups  of  plants,  and  especially  the 
seed  plants,  are  the  ones  which  particularly  and  universally  represent 


THE  FILICALES  291 

the  condition  which  the  supporters  of  this  hypothesis  regard  as 
primitive — that  of  the  tubular  stele  without  internal  phloem  and 
endodermis.  On  the  other  hand,  the  Filicales,  in  .which  the  sipho- 
nostele  commonly  incloses  a  pith  strikingly  resembling  the  tissues 
of  the  cortex  and  quite  generally  lined  inwardly  with  endodermis 
and  phloem,  must  be  regarded  as  more  modern.  The  improbability 
of  this  general  hypothesis  seems  very  great. 

It  may  be  assumed  on  the  basis  of  the  considerations  advanced 
in  earlier  paragraphs  that  the  siphonostelic  condition  in  the  Filicales 
is  susceptible  of  complication  through  the  development  of  medullary 
strands  on  the  one  hand,  and,  on  the  other,  of  simplification 
through  the  loss  of  internal  phloem  and  endodermis  and  the 
progressive  narrowing  of  the  leaf  gaps  resulting  in  sequestration 
of  the  pith.  The  view  that  the  latter  simpler  condition  is  more 
primitive  not  only  runs  counter  to  the  conditions  shown  in  the 
general  sequence  of  types  in  geological  time,  but  is  also  at  variance 
with  the  general  principles  of  comparative  anatomy  detailed  in  a 
former  chapter  of  this  work.  The  hypothesis  that  the  simpler 
condition  of  the  tubular  stele  is  more  primitive  than  the  more 
complex  marks,  moreover,  an  evolutionary  attitude  which  is 
becoming  generally  obsolete  as  our  knowledge  of  the  actual  organ- 
ization of  extinct  forms,  particularly  of  those  types  which  mani- 
fested their  greatest  luxuriance  in  the  past,  becomes  fuller  and 
more  complete.  It  is  now  clear  from  evidence  of  this  kind  that 
the  lower  gymnosperms  have  come  from  the  Filicales  as  a  result 
of  the  simplification  and  reduction  of  the  primary  structures 
of  the  stele  of  the  stem  on  the  one  hand,  accompanied  by  the 
marked  development  of  secondary  fibrovascular  tissues  on  the 
other.  In  the  gymnospermous  Pteropsida  this  anatomical  progress 
has  been  associated  with  the  attainment  of  the  seed  habit.  In 
the  Lycopsida,  by  contrast,  a  similar  anatomical  progress  culmi- 
nated in  much  earlier  geological  times  and  was  not  accompanied 
by  the  evolution  of  true  seeds. 


CHAPTER  XXII 

THE  ARCHIGYMNOSPERMAE:   CYCADOFILICALES 
AND  CYCADALES 

The  earlier  seed  plants  of  Paleozoic  and  Mesozoic  time  were 
characterized  by  the  possession  of  naked  seeds  upon  which  the 
microspores  or  pollen  grains  were  directly  deposited.  The  free 
exposure  of  the  seminal  structures  to  the  air  has  gained  for  the 
types  marked  by  this  feature  the  appellation  of  gymnosperms. 
In  the  large  group  thus  characterized  there  are  two  main  modifi- 
cations. In  the  earlier  and  more  primitive  gymnosperms  the 
pollen  grams  or  microspores  were  accommodated  in  a  chamber  in 
the  apex  of  the  megasporangium  known  as  the  pollen  chamber. 
This  cavity,  filled  with  fluid  derived  either  from  the  surrounding 
parenchymatous  cells  or  in  some  cases  from  a  special  fibrovascular 
system  present  in  the  walls  of  the  megasporangium,  provided 
for  the  germination  of  the  microspores  and  the  subsequent  fertil- 
ization, effected  in  every  instance  by  swimming  sperms.  These 
forms  in  general  present  a  marked  resemblance  to  the  Filicales, 
often  in  external  habit  and  always  in  internal  anatomical  structure. 
In  contrast  to  them  are  the  higher  gymnosperms,  in  which  there 
is  no  true  pollen  chamber.  Here  the  act  of  fertilization  is  effected, 
not  by  swimming  sperms,  but  by  means  of  a  pollen  tube  developed 
as  an  outgrowth  of  the  microspore  and  directed  toward  the  egg. 
The  external  habit  of  these  gymnosperms  is  never  fernlike,  and 
their  anatomical  organization  shows  only  the  slightest  cryptogamic 
features  in  living  forms.  The  types  possessing  pollen  chamber, 
swimming  sperms,  frequent  filicinean  habit,  and  cryptogamic 
organi2^tion  of  the  fibrovascular  structures  are  conveniently 
designated  as  the  Archigymnospermae  or  more  primitive  gymno- 
sperms. Those  forms  which  are  contrasted  to  these  by  the  absence 
of  pollen  chamber  and  the  presence  of  pollen  tube  in  connection 
with  fertilization,  as  well  as  by  the  disappearance  of  filicinean 
habit  and  anatomical  characteristics,  may  be,  on  the  other  hand, 
designated  as  the  Metagymnospermae  or  higher  gymnosperms. 

292 


CYCADOFILICALES  AND  CYCADALES 


293 


The  present  and  the  following  chapter  will  deal  with  the  repre- 
sentatives of  the  Archigymnospermae. 

Our  knowledge  of  the  Archigymnospermae,  from  the  fact 
that  many  of  the  group  are  extinct,  is  necessarily  incomplete. 
It  will  accordingly  be  convenient  in  the  present  chapter  to  focus 
attention  on  the  best-known  fossil  group  and  the  nearly  related 
family  which  possesses  living  representatives.  The  extinct  aggrega- 
tion of  forms  which 
here  present  the 
strongest  claim  to 
attention  are  the 
Cycadofilicales, 
characterized  by  a 
habit  so  closely  re- 
sembling that  of  the 
true  ferns  that  it  is 
only  since  the  very 
end  of  the  last  cen- 
tury that  they  have 
been  recognized  as 
seed  plants.  The 
type  of  seed  con- 
nected with  the  veg- 
etative structures  of 
the  Cycadofilicales 
has  been  diagrammatically  indicated  in  an  earlier  chapter,  to  which 
the  reader  is  here  referred.  In  the  present  connection  only  ana- 
tomical features  of  the  group  need  be  discussed.  The  antiquity 
of  the  Cycadofilicales  is  vouched  for,  not  only  by  their  Paleozoic 
occurrence,  but  also  by  the  occasional  presence  of  protostelic 
stems,  axes  of  this  type  not  being  known  for  any  other  group 
of  seed  plants  living  or  extinct.  Fig.  207  illustrates  the  organiza- 
tion of  the  protostelic  axis  of  the  genus  Heterangium  from  the 
English  Carboniferous.  The  transverse  section  shows  the  presence 
of  an  external  ribbing  of  sclerenchymatous  strands,  a  feature  com- 
mon to  many  older  representatives  of  the  Gymnospermae.  The 
fibrovascular  apparatus  consists  of  protostelic  primary  wood  sur- 
rounded by  a  thin  layer  of  secondary  xylem. 


FIG.  207. — Transverse  section  of  the  stem  of  Heter- 
angium (after  Scott) . 


294  THE  ANATOMY  OF  WOODY  PLANTS 

We  may  now  turn  our  attention  to  more  complicated  stems 
belonging  to  the  general  group  of  the  Cycadofilicales.  The  genus 
Medullosa  is  of  particular  importance,  since  it  possesses  a  type  of 
stelar  organization  presented  by  several  or  many  concentric  strands 
which  are  best  considered  as  belonging  to  a  siphonostelic  cylinder. 
The  general  organization  of  the  axis  in  the  genus  under  discussion 
is  well  shown  in  Fig.  208,  which  portrays  in  a  somewhat  diagram- 
matic fashion  the  structures  involved.  There  are  three  large 
concentric  fibrovascular  strands  present  which  consist  of  a  central 
core  of  primary  wood  indicated  by  cross-hatching,  surrounded 


LAB. 

FIG.  208. — Transverse  section  of  the  stem  of  Medullosa  anglka  (after  Scott) 

with  a  cordon  of  secondary  xylem,  represented  by  radiating  lines. 
The  outline  of  the  stem  is  irregular,  the  salients  being  due  to  the 
presence  of  the  bases  of  large  leaves.  The  surface  is  covered  with 
the  reticulated  sclerenchymatous  strands  already  described  in  the 
case  of  Eeterangium.  The  fundamental  tissues  are  occupied  by 
secretory  canals,  which  probably  contained  mucilage,  as  do  those 
of  the  living  Cycadales.  In  the  cortical  region  are  accommodated 
larger  and  smaller  strands  ordinarily  lacking  secondary  growth. 
The  bundles  of  greater  dimensions  are  mostly  the  larger  foliar 
bundles  and  possess  concentric  organization.  The  small  strands 
which  are  found  characteristically  in  the  actual  leaf  bases  are 
collateral  in  their  organization  and  exarch  in  the  structure  of  their 
xylem.  Fig.  209  illustrates  two  of  the  smaller  strands  together 
with  fundamental  tissues  and  contained  mucilage  canals.  The 
general  situation  in  the  genus  Medullosa  is  complicated  frequently 


CYCADOFILICALES  AND  CYCADALES 


295 


by  the  presence  of  more  numerous  large  concentric  fibrovascular 
strands  than  those  shown  in  Fig.  208.  In  such  instances  the  con- 
centric strands  often  show  a  more  or  less  complete  degeneracy  of 
the  secondary  xylem  on  their  inner  surfaces.  This  situation  has 
been  regarded  with  reason  as  of  significance  in  foreshadowing  the 
type  of  organization  found  in  the  axes  of  the  Cycadales,  living 
and  extinct.  It  will  be  seen  from  the  account  of  Medullosa  here 
supplied  that  the  genus  presents  some  marked  features  of  ana- 
tomical resemblance  to 
ferns,  the  main  contrasts 
in  organization  being  due 
to  the  appearance  of 
secondary  growth  in  con- 
nection with  the  strands 
belonging  to  the  stem 
proper.  In  accordance 
with  the  canons  of  anat- 
omy formulated  in  an 
earlier  chapter  of  the  pres- 
ent work,  the  secondary 
activity  has  not  yet  pene- 
trated into  the  traces  which 
pass  out  into  the  more  con- 
servative foliar  structures.  The  Medulloseae  are  undoubtedly  of 
great  interest  from  the  standpoint  of  the  evolution  of  cycadean 
forms,  and  there  can  be  little  question  that  these  types  so  common 
as  charred  remains  hi  the  Carboniferous  coals  of  Europe  and 
America  came  very  near  to  being  the  actual  ancestors  of  our 
living  Cycadales. 

As  a  third  illustration  of  the  Cycadofilicales  we  may  take  the 
genus  Lyginodendron,  the  organization  of  which  has  been  so  admir- 
ably described  by  English  anatomists.  Fig.  210  illustrates  the 
structure  of  the  stem  in  this  genus.  The  same  sclerined  ribbing 
is  observed  as  in  the  other  two  genera  discussed  above.  The 
central  cylinder,  however,  presents  a  marked  contrast  to  that  of 
either  Heterangium  or  Medullosa,  for  it  consists  of  collateral  strands 
arranged  in  a  closed  circle.  The  pith  is  occupied  by  sclerotic 


FIG.  209. — Foliar  bundles  of  Medullosa  angltca 


296 


THE  ANATOMY  OF  WOODY  PLANTS 


nests  of  cells  which  are  duplicated  by  similar  structures  in  the 
cortex.  The  inner  surface  of  the  cylinder  of  xylem  shows  clusters 
of  primary  wood  which  sharply  contrast  with  the  secondary 
region  by  the  irregularity  of  the  arrangement  of  the  elements  and 
the  absence  of  rays.  Fig.  211  illustrates  a  portion  of  the  cylinder 
more  highly  magnified,  and  with  the  greater  enlargement  the 
distinct  and  mesarch  character  of  the  primary  region  becomes 
apparent.  The  secondary  wood  is  characterized  both  by  its 
regularly  radial  arrangement  and  by  the  presence  of  rays  which 


FIG.  210.— Transverse  section  of  the  stem  of  Lyginodendron  oldhamium  (after 
Scott). 

are  often  two  or  three  cells  in  width.  The  tracheids  of  secondary 
origin  are  marked  by  crowded  pits  which  alternate  in  position 
and  are  often  angular  from  mutual  contact.  These  pores  are 
confined  strictly  to  the  radial  walls  of  the  elements,  no  tangential 
pitting  occurring  in  the  secondary  tracheids  of  any  Paleozoic 
gymnosperms  with  which  we  are  at  the  present  time  acquainted. 
The  foliar  traces  in  Lyginodendron  are  at  first  single,  but  bifurcate 
shortly  after  leaving  the  primary  region  of  the  stem,  and  lose 
their  secondary  xylem,  becoming  concentric  instead  of  collateral 
in  their  organization.  It  is  evident  on  the  basis  of  the  general 
canons  earlier  elucidated  that  the  bundles  of  the  stem  in  the 
case  of  Lyginodendron  must  formerly  have  been  concentric  in 
their  structure,  since  this  condition  persists  in  the  traces  of  the 
leaves.  Further,  the  presence  of  secondary  wood  in  the  stem 


CYCADOFILICALES  AND  CYCADALES 


297 


and  its  absence  in  the  foliar  traces  show  that  secondary  growth 
is  a  comparatively  recently  acquired  feature  in  the  genus  under 
discussion.  The  sclerotic  nests  in  the  pith  appear  to  vouch  for 
the  extra-stelar  origin  of  the  medullary  region.  The  situation 
as  a  whole  is  like  that  found  in  the  case  of  the  Osmundaceae, 
except  for  the  complication  introduced  by  the  presence  of  the 
secondary  growth.  In  the  genus  Botrychium  among  the  Ophio- 
glossaceae  secondary  tissues,  however,  are  often  well  developed, 
and  it  has  long 
been  realized  that 
the  occurrence  of 
secondary  activity 
in  the  fibrovascular 
tissues  is  not  an 
extremely  impor- 
tant criterion  from 
the  evolutionary 
standpoint.  It 
was  at  one  time 
thought  that  the 
genus  Lyginoden- 
dron  was  the  proto- 
type  of  the 
Cycadales,  but  this 
opinion  has  been 
given  up  in  favor 
of  a  derivation  of 

the  cycadean  type  from  forms  like  the  Medulloseae,  for  which  the 
evidence  is  extremely  strong. 

We  may  now  turn  our  attention  to  the  anatomical  organization 
of  that  group  of  gymnosperms  which  still  persists  as  the  Cycadales. 
This  family  was  well  developed  in  Mesozoic  times,  but  there  is 
no  evidence  that  it  was  represented  in  the  Paleozoic  age.  The 
stem  of  the  cycadean  gymnosperms,  in  its  modern  representatives 
at  any  rate,  is  soft  and  parenchymatous  and  is  characterized  by  a 
very  large  medulla.  Both  cortex  and  medulla  contain  numerous 
mucilage  canals.  The  woody  cylinder  is  ordinarily  very  thin 


FIG.  211. — Transverse  section  of  primary  and  second- 
ary wood  in  Lyginodendron. 


298  THE  ANATOMY  OF  WOODY  PLANTS 

and  in  the  vegetative  axes  does  not  manifest  the  presence  of 
cryptogamic  or  centripetal  wood.  In  certain  genera  of  the  Cyca- 
dales,  both  living  and  extinct,  the  woody  cylinder  often  shows  a 
curious  reduplication,  resulting  from  the  appearance  of  new  circles 
of  fibro vascular  tissue  in  the  pericycle.  The  explanation  of  these 
structures  is  somewhat  difficult.  One  view  is  that  they  represent 
a  persistence  of  the  numerous  cycles  of  fibrovascular  strands  in 
certain  types  of  Medulloseae.  This  view,  however,  is  rendered 
difficult  by  a  situation  which  often  presents  itself  in  the  Cycadales. 
In  the  region  of  the  stem  where  a  cone  is  attached  the  medulla  is 
frequently  occupied  by  numerous  strands  which  are  absent  else- 
where. It  is  natural  to  regard  such  structures  as  representing  a 
return  of  the  medullosan  organization  in  relation  to  the  attachment 
of  conservative  reproductive  axes.  If  this  explanation  be  adopted 
for  the  medullary  bundles,  the  interpretation  of  the  supernumer- 
ary zones  of  fibrovascular  tissue  formed  successively  in  the  peri- 
cycle  of  the  original  cylinder  cannot  be  accepted.  A  more  probable 
elucidation  of  the  situation  is  furnished  by  the  habit  of  certain 
Mesozoic  representatives  of  the  group  which  are  contrasted  to 
modern  forms  by  their  slender  and  freely  branching  habit.  This 
condition,  for  example,  is  found  in  the  genus  Anomozamites.  It  is 
not  improbable  that  the  genus  mentioned  was  a  climbing  plant. 
The  formation  of  supernumerary  zones  of  fibrovascular  tissue  is 
a  common  feature  of  organization  of  the  stem  in  climbing  plants 
of  widely  diverse  gymnospermous  and  dicotyledonous  affinities. 
It  is  consequently  not  unlikely  that  the  Mesozoic  forebears  of  living 
Cycadales  developed  successive  zones  of  fibrovascular  tissues  in 
their  stems  as  a  result  of  a  climbing  habit.  After  this  feature  had 
become  thoroughly  fixed  by  the  lapse  of  long  geological  time,  a 
desert  habit  was  superimposed  on  it  with  a  resulting  anatomical 
organization  such  as  is  found  in  the  living  representatives  of  the 
group.  A  similar  hypothesis  has  to  be  invoked,  as  will  be  shown 
in  a  subsequent  chapter,  to  explain  the  remarkable  anatomical 
resemblance  between  the  desert-inhabiting  gnetalian  genus  Welwit- 
schia  and  the  vinelike  Gnetum. 

Although  there  is  no  instance  of  the  presence  of  centripetal 
or  cryptogamic  wood  in  the  vegetative  axis  of  the  Cycadales, 


CYCADOFILICALES  AND  CYCADALES  299 

not  a  few  examples  of  the  existence  of  xylem  of  this  type  have 
been  described  by  Scott  in  the  reproductive  axes  or  cones.  In 
Fig.  212  is  shown  a  transverse  section  of  part  of  the  peduncle  of 
the  female  cone  of  Stangeria  paradoxa.  Although  the  wood  as  a 
whole  is  strongly  centrifugal  or  peripheral  in  its  development,  a 


FIG.  212.— Transverse  section  of  one  of  the  strands  of  the  peduncle  of  the  cone 
in  Stangeria  paradoxa. 

small  amount  is  formed  over  against  the  medullary  region.  This 
condition  is  a  persistence  of  the  centripetal  wood  of  the  Paleozoic 
Cycadofilicales,  from  which  the  Cycadales  have  in  all  probability 
been  derived.  The  anatomical  principle  here  involved  is  one  of 
great  importance  and  was  first  clearly  emphasized  by  Scott. 
Subsequent  to  his  observations  on  the  Cycadales,  it  has  been 


300  THE  ANATOMY  OF  WOODY  PLANTS 

discovered  to  be  exemplified  in  a  remarkable  manner  by  the 
observation  of  the  wood  in  the  axes  of  members  of  the  Conifer- 
ales,  as  will  be  noted  at  a  later  stage. 

The  leaves  of  the  Cycadales  are  of  great  importance  from  the 
standpoint  of  evolutionary  anatomy.  Here  the  centripetal  wood, 
which  is  poorly  developed  at  best  in  reproductive  axes,  is  abundant, 
well  defined,  and  universal  in  its  occurrence.  Fig.  213  illustrates 
the  transverse  section  of  a  leaflet  of  Cycas  revoluta.  It  is  apparent 


FIG.  213. — Transverse  section  of  a  foliar  strand  in  Cycas  revoluta 

that  the  mass  of  the  xylem  is  of  primary  origin  and  spreads  out 
in  a  fanlike  fashion  toward  the  upper  surface  of  the  leaf.  It  is 
therefore  centripetal  in  its  development.  Below  the  mass  of 
centripetal  xylem  lies  a  much  more  scanty  development  in  the 
region  of  the  regularly  arranged  phloem.  This  is  the  centrifugal 
xylem,  which  is  partially  primary  and  partially  secondary  in  its 
origin.  Fig.  214  illustrates  the  longitudinal  section  of  a  leaf 
trace  in  Cycas.  The  phloem,  easily  recognized  by  its  characteristic 
sieve  tubes,  is  seen  toward  the  right.  To  the  left  is  situated  the 
centripetal  wood,  composed  from  left  to  right  of  pitted,  scalariform, 
reticulate,  and  spiral  elements,  the  latter  constituting  the  protoxy- 


CYCADOFILICALES  AND  CYCADALES 


301 


lem.  After  a  narrow  parenchymatous  interval  the  pitted  elements 
of  the  centrifugal  metaxylem  are  seen  in  close  proximity  to  the 
phloem.  The  general  situation  in  the  transverse  and  longitudinal 
sections  of  the  foliar  strands  shown  in  the  two  figures  is  universal 
for  the  Cycadales,  any  minor  divergences  which  have  been  ob- 
served being  mainly  the  result  of  the  smaller  size  of  the  strands  in 
the  genera  other  than  Cycas. 


FIG.  214. — Longitudinal  section  of  a  foliar  strand  in  Cycas  revoluta 


The  foliar  traces  in  the  genus  under  consideration  show  other 
interesting  features  of  organization  in  the  lower  part  of  their 
course,  where  they  pass  from  the  base  of  the  petiole  of  the  leaf 
into  the  cortex  of  the  axis.  These  features  have  to  do  both  with 
the  arrangement  of  the  traces  and  with  their  anatomical  organiza- 
tion. In  the  vegetative  axes  of  living  genera  of  the  Cycadales 
the  foliar  strands,  instead  of  passing  directly  outward  to  their 
respective  leaves,  pursue  a  meandering  course  through  the  cortex 
and  are  known  as  " girdles."  This  condition  is  confined  to  the 
modern  genera,  for  in  the  Mesozoic  representatives  of  the  group 
united  under  the  caption  of  Bennettitales  the  leaf  traces  pass 
directly  to  their  corresponding  leaves.  Interestingly  enough  this 


302 


THE  ANATOMY  OF  WOODY  PLANTS 


condition  is  paralleled  in  the  reproductive  axes  of  the  cones  of 
the  living  Cycadales  and  in  the  seedling  of  their  most  primitive 
genus,  Cycas. 

As  regards  anatomical  organization  the  foliar  traces  in  their 
cortical  course,  and  even  sometimes  in  the  bases  of  the  petioles, 
show  an  interesting  resemblance  to  the  bundles  of  the  stem  in 
Medullosa.  Fig.  215  illustrates  the  structure  of  one  of  the  leaf 

traces  of  Cycas  re- 
valuta  as  it  passes 
through  the  cortex. 
The  bundle  is 
clearly  concentric 
and  consists  of  a 
central  core  of 
primary  wood  sur- 
rounded by  regu- 
larly  seriate 
secondary  xylem 
and  traversed  by 
wood  rays.  If  the 
figure  under  discus- 
sion be  compared 
with  that  of  a  stem 
bundle  of  Medul- 
losa  (Fig.  208),  it 
becomes  evident  that  the  only  striking  difference  is  presented  by 
the  much  smaller  size  of  the  strand  of  the  living  genus. 

The  root  in  the  Cycadales  presents  little  that  is  of  interest, 
since  the  feature  of  the  presence  of  centripetal  wood  is  equally 
exemplified  by  all  roots  as  a  characteristic  connected  with  their 
extremely  conservative  organization.  It  is  only  between  Mesozoic 
and  modern  types  in  which  the  evolutionary  importance  of  the 
centripetal  wood  has  passed  into  the  background  that  the  anatom- 
ical structure  of  the  root  comes  into  the  phylogenetic  foreground 
in  connection  with  the  doctrine  of  descent.  A  feature  not  of 
evolutionary  interest  presented  by  the  cycadean  root  is  the  frequent 
presence  of  root  tubercles. 


FIG.  215. — Cortical  strand  of  Cycas  revoluta 


CYCADOFILICALES  AND  CYCADALES  303 

The  Mesozoic  Cycadales  are  ordinarily  grouped  under  the 
heading  Bennettitales  on  account  of  the  remarkable  features  pre- 
sented by  their  reproductive  structures,  which  are  very  different 
from  those  exemplified  by  any  living  Cycadales.  The  uniting  of 
both  microsporophylls  and  megasporophylls  in  the  same  strobilus 
is  a  frequent  feature  of  this  group  and  has  been  considered  by  some 
to  indicate  an  affinity  with  the  angiosperms,  particularly  as  the 
ovuliferous  sporophylls  are  shut  in  by  a  panoply  provided  by  the 
swollen  apices  of  so-called  interseminal  scales — sterile  structures 
interposed  among  the  fertile  ones.  The  question  of  the  possible 
descent  of  the  angiosperms  from  the  Bennettitales  is  one  which 
can  scarcely  be  considered  seriously  from  the  anatomical  standpoint, 
since  there  is  practically  nothing  in  common  in  the  anatomical 
organization  of  the  two  groups,  either  in  the  reproductive  or  in 
the  vegetative  features.  The  fructifications  of  the  Bennettitales 
before  their  real  affinities  were  known  were  attributed  by  the 
distinguished  French  paleobotanist  Saporta  to  a  group  which  he 
designated  as  proangiosperms.  Now  that  their  actual  relation- 
ships are  known,  there  seems  scarcely  any  reason  for  regarding 
them  as  allied  to  the  angiosperms.  The  Bennettitales  supply  an 
excellent  illustration  of  the  relative  conservatism  of  anatomical 
structures,  for,  although  they  present  marked  differences  in  repro- 
ductive features  from  the  Cycadales,  their  vegetative  anatomy 
does  not  differ  in  any  important  particular  from  that  of  the  modern 
group. 

If  the  anatomical  situation  for  the  Cycadales  in  the  large 
sense  be  summarized,  it  becomes  clear  on  the  basis  of  the  general 
canons  of  comparative  anatomy  elucidated  in  earlier  pages  of 
the  present  volume  that  they  exhibit  strong  filicinean  features. 
The  anatomical  examination  of  the  foliar  strands  reveals  crypto- 
gamic  or  centripetal  wood  and  in  some  cases  concentric  organiza- 
tion. The  evidence  derived  from  the  consideration  of  the  anatomy 
of  the  leaf  is  confirmed  in  a  number  of  instances  by  the  organiza- 
tion of  the  strands  of  xylem  in  the  reproductive  axis.  The  testi- 
mony of  the  conservative  parts  leads  to  the  assumption  of  the 
former  presence  of  centripetal  xylem  and  concentric  fibrovascular 
strands  in  the  vegetative  stem.  The  general  result  reached  on 


304  THE  ANATOMY  OF  WOODY  PLANTS 

the  basis  of  comparative  anatomy  receives  strong  support  from 
the  organization  of  certain  Paleozoic  Cycadonlicales,  notably  the 
Medulloseae.  There  seems  to  be  little  doubt  that  the  genera  of 
Cycadales  in  the  broadest  sense  of  the  term  have  been  derived 
from  ancestors  closely  resembling  vegetatively  the  Medullosa 
type.  It  follows  that  so  far  as  their  primary  fibrovascular  struc- 
tures are  concerned  they  present  a  condition  of  reduction  from 
the  anatomical  organization  found  in  the  oldest  and  most  fernlike 
gymnosperms. 


CHAPTER  XXIII 

THE  ARCHIGYMNOSPERMAE:   CORDAITALES 
AND  GINKGOALES 

The  two  tribes  of  gymnosperms  discussed  in  the  present  chapter 
no  longer  possess  the  fernlike  habit  of  the  Archigymnospermae 
which  have  just  been  considered,  but  still  maintain  the  pollen 
chamber  and  the  cryptogamic  wood  so  characteristic  of  the  Cyca- 
dofilicales  and  Cycadales.  Of  the  two  groups  here  discussed  the 
Cordaitales  became  entirely  extinct  at  the  end  of  the  Paleozoic 
or  at  the  beginning  of  the  Mesozoic  and  have  a  geological  range 
extending  far  into  the  past.  On  the  other  hand,  the  Ginkgoales, 
although  present  somewhat  doubtfully  in  the  Paleozoic,  had  their 
greatest  development  in  the  Mesozoic  and  persist  at  the  present 
time  in  a  monotypic  genus  of  Eastern  Asia. 

The  Cordaitales  were  freely  branched  trees  resembling  in 
general  habit  our  existing  conifers,  to  which  they  are  usually 
considered  to  have  given  origin.  The  trunks  often  reached  a 
considerable  size,  and  the  woody  cylinder  included  a  large  pith, 
in  some  instances  septate  and  occupied  by  air  spaces,  which, 
however,  unlike  the  medullary  fistulae  of  Catamites,  were  much 
more  numerous  than  the  internodes.  The  cavernous  character 
of  the  pith  as  well  as  its  large  size  is  responsible  for  the  existence 
of  medullary  casts  in  the  case  of  the  Cordaitales,  and  these  are 
assembled  under  the  form-genus  Artisia  or  Sternbergia.  The 
surface  of  the  stem  was  often  provided  with  the  sclerenchymatous 
armor  which  has  been  noted  as  a  feature  of  organization  of  the 
axes  of  the  Cycadofilicales.  Fig.  216  illustrates  the  general 
topography  of  a  segment  of  the  stem  in  a  cordaitean  form.  The 
woody  cylinder  was  without  annual  rings,  except  in  the  later 
Paleozoic  and  hi  higher  latitudes.  This  situation  is  very  well 
shown  by  Figs.  217  and  218,  illustrating  the  structure  of  cordaitean 
woods  from  Northern  England  (Lancashire)  and  Southern  Canada 
(Prince  Edward  Island).  In  the  first  specimen  annual  increments 
can  be  somewhat  clearly  distinguished,  while  in  the  second  they 

3°S 


3o6 


THE  ANATOMY  OF  WOODY  PLANTS 


are  conspicuous  by  their  absence.  Although  some  indication  of 
the  existence  of  annual  diversities  of  temperature  is  supplied 
by  certain  cordaitean  stems,  the  organization  of  the  annual  ring 
in  such  cases  is  extremely  simple,  and  even  the  differentiation 
involved  in  the  presence  of  terminal  tangential  pitting  is  not  seen, 
as  may  be  ascertained  by  reference  to  Fig.  219,  representing  a 
specimen  from  Lancashire,  England,  in  stereoscopic  view  under 
a  high  magnification.  It  is  apparent  from  the  figures  that  the 


US 


FIG.  216. — Part  of  a  transverse  section  of  a  cordaitean  stem  (after  Scott) 

secondary  xylem  of  Cordaitales  was  of  very  simple  structure  and 
that  the  rays,  unlike  those  of  the  two  tribes  considered  in  a 
previous  chapter,  were  usually  a  single  row  of  cells  in  width. 
This  narrowness  of  the  strands  of  radial  parenchyma  is  shared 
by  the  Cordaitales  with  the  Ginkgoales  and  the  conifers.  The 
wood,  like  that  of  other  Paleozoic  gymnosperms,  was  entirely 
without  longitudinal  parenchymatous  elements,  and  the  bor- 
dered pits  were  strictly  confined  to  the  radial  walls  of  the 
tracheids. 

Another  interesting  feature  of  the  organization  of  the  xylem 
in  cordaitean  forms  was  the  extremely  long  region  of  transition 
between  the  spiral  elements  of  the  protoxylem  and  the  first  pitted 


CORDAITALES  AND  GINKGQALES 


307 


elements  of  the  secondary  wood.     This  is  well  shown  in  Fig.  220. 

Among  the  living  conifers  this  condition  is  most  nearly  paralleled 

by  the  Abietineae. 
The  foliar  trace 

is  of  course  of  great 

importance    in    this 

as  in  other  ancient 

gymnosperms. 

Fig.    221    illustrates 

the  organization  of 

one  of  the  bundles 

of  a   broad   cordai- 

tean  leaf  (Cordaites 

principalis)  in  both 

transverse  and 

longitudinal  section. 

In  a  is  shown   the 

transverse    topog- 

raphy  of  the  strand, 

and  it  is  clear  that  the  centripetal  wood  is  well  developed,  ending 

upwardly  in  large 
elements  which  are 
pitted  in  their  char- 
acter. In  the  par- 
ticular instance 
figured  there  hap- 
pens to  be  no  devel- 
opment  of  the 
centrifugal  xylem,  so 
that  the  phloem 
abuts  immediately 
on  the  protoxylem. 
With  the  flanks  of 
the  metaxylem  is 
connected  a  zone  of 
narrow  thick-walled 

FIG.  218— Cordaitean  wood  from  Prince  Edward  Island    elements  which  form 


308  THE  ANATOMY  OF  WOODY  PLANTS 

a  cordon  completely  around  the  phloem.  Outside  the  cells  of  nar- 
row lumen  is  found  a  second  zone,  composed  of  elements  larger  in 
diameter.  Both  outer  and  inner  zones  consist  of  tracheary  elements 


FIG.  219. — Stereoscopic  view  of  a  cordaitean  wood  from  Lancashire,  England 

and  constitute  the  so-called  double  transfusion  sheath.  In  b,  the 
longitudinal  view,  the  various  elements  of  the  bundle  are  shown 
in  the  median  section.  The  protoxylem  is  continuous  with  tra- 
cheids  which  by  the  usual  transitions  gradually  pass  into  the 


CORDAITALES  AND  GINKGOALES 


3°9 


pitted  elements  of  the  last-formed  metaxylem.  Below  the  pro- 
toxylem  lies  the  phloem,  and  still  farther  down  the  more  elon- 
gated and  narrower  elements  of  the  inner  transfusion  sheath,  which 
in  turn  abut  on  the  short,  broad  tracheary  elements  of  the  outer 
transfusion  sheath.  At  the  very  top  lie  other  short  transfusion 
cells,  and  the  inner  elongated  sheath  in  this  region  is  absent  as  a 
result  of  conditions  which  can  readily  be  inferred  from  the  in- 
spection of  the  transverse  view  in  a.  There  is  some  variety  in 
the  development  of  the  foliar  bundles  of  the  Cordaitales,  but 
all  are  characterized  by  the  presence  of  well-marked  centripetal 
wood  and  a  cordon  of  short  tracheary  elements,  known  as  trans- 


FIG.  2  20. — Longitudinal  view  of  cordaitean  wood  near  the  pith  (after  Scott) 

fusion  cells,  which  are  closely  related  to  the  centripetal  or  crypto- 
gamic  wood. 

The  root  in  cordaitean  forms,  for  reasons  applying  equally 
to  all  Paleozoic  gymnosperms,  presents  no  features  of  special 
interest  beyond  illustrating  the  general  cordaitean  organization 
modified  to  the  needs  of  root  organs. 

It  will  be  obvious  from  the  statements  made  in  the  foregoing 
paragraphs  that  there  is  clear  evidence  in  the  organization  of 
the  foliar  structures  in  the  Cordaitales  for  their  close  affinity  with 
the  Filicales,  although  naturally  the  degree  of  relationship  is 
much  less  intimate  than  that  which  characterizes  the  Cycadofili- 
cales  and  even  the  Cycadales.  Concerning  the  organization  of 
the  microsporangia  and  seeds  of  the  Cordaitales  our  knowledge 
is  unfortunately  somewhat  meager.  The  evidence  in  regard  to 
the  microsporangium  is  not  sufficiently  definite  to  warrant  an 
opinion  as  to  whether  it  was  ectokinetic  or  endokinetic  in  its 


3io 


THE  ANATOMY  OF  WOODY  PLANTS 


mode  of  dehiscence;  but,  in  view  of  the  strong  development  of 
transfusion  tissue  in  the  foliar  organs  of  the  group  under  discus- 
sion, a  clear  feature  of  distinction  from  the  Cycadales  and  Cyca- 
dofilicales  (in  both  of  which  the  microsporangia  are  ectokinetic), 
it  is  somewhat  probable  that  the  pollen  sacs  owed  their  dehiscence 
to  a  layer  of  tracheary  origin.  The  seeds  of  Cordaites  have  been 


FIG.  221. — Transverse  and  longitudinal  sections  of  a  leaf  bundle  in  Cordaites 
principalis. 

anatomically  investigated  by  Renault  and  they  possessed  a  well- 
marked  pollen  chamber.  A  more  complete  knowledge  of  the 
reproductive  structures  of  cordaitean  forms  and  of  the  types 
which  connected  them  in  the  more  remote  Paleozoic  with  filicinean 
ancestors  is  much  to  be  desired. 

The  Ginkgoales  are  represented  by  a  single  living  genus,  but 
were  extremely  abundant  in  the  Mesozoic  and  are  thought  to 
have  been  continued  into  the  Paleozoic  by  the  somewhat  problem- 
atical genus  Whittleseya.  Unfortunately  our  anatomical  knowledge 
of  the  group  beyond  that  supplied  by  the  investigation  of  the 


CORDAITALES  AND  GINKGOALES  311 

living  genus  is  extremely  meager  and  in  fact  is  confined  to  the 
structure  of  woods  which  have  been  referred  to  the  group. 

The  stem  in  Ginkgo  is  characterized  by  the  presence  of  clear 
annual  rings  which  terminate  with  tracheids  provided  with  tan- 
gential pits  and  in  this  respect  reveal  a  marked  contrast  to  the 
tracheary  elements  constituting  the  remainder  of  the  annual 
increment.  It  is  obvious  that  as  regards  the  organization  of  the 
annual  ring  the  group  under  discussion  is  distinct  from  those  rare 
cordaitean  stems  in  which  yearly  zones  of  growth  can  be  dis- 
tinguished, by  the  presence  of  tangential  terminal  pitting.  In 
other  respects,  however,  the  structure  of  the  wood  is  clearly  archaic, 
for  there  are  no  parenchymatous  elements  present  other  than  those 
related  to  the  rays.  The  pith  and  cortex  in  the  group  possess 
secretory  canals  which  are  comparable  to  those  found  in  certain 
Abietineae. 

The  longitudinal  aspect  of  the  secondary  xylem  in  Ginkgo 
is  very  different  from  that  of  the  Cordaitales.  In  the  more  ancient 
group  the  radial  pits  are  often  extremely  numerous  and  they  are  then 
angular  by  mutual  contact.  In  Ginkgo  the  pores  of  the  tracheids 
are  not  so  abundant  as  to  be  described  as  crowded  and,  moreover, 
instead  of  being  alternating  and  angular  as  in  the  older  tribe 
are  round  and  opposite.  Another  equally  striking  feature  offers 
itself  in  the  presence  of  transverse  bars  of  pectic  cellulose  in  the 
walls  of  the  tracheids  between  the  pairs  of  opposite  pits.  These 
may  conveniently  be  designated  bars  of  Sanio,  to  distinguish 
them  from  the  trabeculae  of  Sanio,  structures  which  are  found 
not  uncommonly  in  all  woods  of  secondary  origin  from  (and 
including)  those  of  the  Cycadales  to  those  of  the  dicotyledons. 
The  latter  structures  consist  of  ligneous  processes  crossing  the 
cavity  of  the  tracheid,  possibly  due  to  the  activity  of  parasitic 
fungi,  while  the  true  bars  of  Sanio  are  concealed  in  the  wall  itself 
and  consist  of  pectic  cellulose.  Bars  of  Sanio  are  found  only  in 
the  walls  of  tracheids  of  secondary  origin,  and  statements  as  to 
their  occurrence  in  any  elements  of  the  primary  wood  are  erroneous. 

An  interesting  condition  appears  in  the  organization  of  the 
secondary  wood  of  the  peduncle  of  the  seed.  In  Fig.  222  is 
shown  the  transition  region  in  the  xylem.  It  will  be  observed 


3I2 


THE  ANATOMY  OF  WOODY  PLANTS 


that  the  pitted  tracheids  nearest  to  the  primary  wood  are  entirely 
without  bars  of  Sanio,  which  make  their  appearance  only  at  an 
interval  from  the  protoxylem.  The  pitting  to  a  large  extent  is 
alternate.  It  is  very  generally  admitted  by  competent  judges 
at  the  present  time  that  the  Ginkgoales  are  derived  from  cordaitean 
ancestry,  and  it  is  accordingly  highly  interesting  to  find  in  the 


•^  © 

©0 


I 


FIG.  222.— Longitudinal  view  of  the  tracheids  in  the  peduncle  of  a  seed  in  Ginkgo. 
To  the  right  is  shown  the  arrangement  of  the  tracheids  in  the  mature  wood. 


organization  of  vegetative  and  reproductive  axes  evidence  based 
on  the  pitting  and  distribution  of  the  bars  of  Sanio  favorable  to 
such  an  opinion.  Farther  away  from  the  primary  wood  the 
secondary  tracheids  quickly  develop  the  opposite  pitting  and 
bars  of  Sanio  characteristic  of  the  mature  wood.  It  is  clear  from 
the  figure,  moreover,  that  the  tracheids  of  the  primary  wood  are 
quite  devoid  of  structures  of  the  nature  of  the  bars  of  Sanio.  It 
is  well  to  emphasize  the  conditions  found  in  the  organization  of 
the  root  and  reproductive  axis  of  Ginkgo,  because  there  prevails 


CORDAITALES  AND  GINKGOALES  313 

at  the  present  time  an  almost  inexcusable  ignorance  in  regard  to 
the  nature  and  distribution  of  the  structures  here  designated 
bars  of  Sanio.  They  are  clearly  correlated  with  opposite  pitting 
and  are  a  feature  of  the  secondary  wood,  not  appearing  in  the 
organization  of  the  tracheids  of  the  primary  xylem.  Evidently 
the  structures  in  question  are  of  considerable  value  in  the  identi- 
fication of  gymnospermous  woods  and  consequently  must  rank 
high  as  a  diagnostic  criterion  among  competent  anatomists. 
The  mature  vegetative  leaf  in  Ginkgo  supplies  very  little  evidence 


FIG.  223. — Foliar  bundle  of  Ginkgo,  showing  transfusion  tissue  (after  Sprecher) 

of  the  presence  of  centripetal  elements  in  the  strict  sense  of  the 
term.  In  the  terminal  region  of  the  blade  of  the  leaf  a  well- 
marked  zone  of  transfusion  tissue  manifests  itself,  as  is  shown  in 
Fig.  223;  but  typical  centripetal  tracheids  are  usually  conspicuous 
by  their  absence.  In  the  cotyledon,  however,  the  centripetal  or 
cryptogamic  wood  is  present  in  a  much  clearer  manner  in  accord- 
ance with  the  principle  of  recapitulation  discussed  in  an  earlier 
chapter.  The  reproductive  leaves,  both  ovuliferous  and  staminate, 
also  show  the  centripetal  elements  in  a  good  condition  of  develop- 
ment, although  even  here  they  more  nearly  resemble  transfusion 
tissue.  In  the  stalk  which  supports  the  pair  of  ovules  centrip- 
etal elements  and  ordinary  transfusion  cells  are  seen  in  the  upper 
region  in  great  abundance  and  are  likewise  found  in  the  collar  sur- 
rounding the  base  of  the  seeds.  The  situation  presented  by  the 


THE  ANATOMY  OF  WOODY  PLANTS 


microsporophyll  is,  however,  of  greater  interest  in  the  present 
connection.  In  the  petiole  of  the  bisporangiate  microsporophyll 
tracheary  elements  of  a  centripetal  character  occur  on  the  upper 
side  of  the  protoxylem  (Fig.  224).  These  elements  can  scarcely 
be  said  to  constitute  typical  centripetal 
tracheids,  since  they  are  often  of  wide 
lumen  and  are  correspondingly  abbrevi- 
ated in  length.  As  the  foliar  traces 
ascend  into  proximity  to  the  sporangia, 
they  separate  from  one  another  and  the 
xylem  of  each  rotates  so  as  to  occupy  a 
position  near  the  middle  line  of  the 


a  b 

FIG.  224. — (a)  longitudinal,  (b)  transverse,  section  of  wood  of  bundle  in  micro- 
sporophyll of  Ginkgo. 

sporophyll,  while  the  strands  of  phloem  turn  outward,  to  end  in  the 
bases  of  the  sporangia.  Meanwhile  the  transfusion  elements  occur- 
ring on  the  upper  side  of  the  tracheary  strands  in  their  upward 
course  pass  imperceptibly  into  the  fibrously  thickened  mechanical 
elements  which  are  responsible  for  the  dehiscence  of  the  sporangium. 
Further,  the  apex  of  the  tracheary  strands  passes  gradually  by 
means  of  short  transfusion  tracheids  into  the  mechanical  elements 
which  lie  along  the  median  sides  of  the  sporangia.  In  this  fashion 
there  is  established  an  intimate  relation  between  the  tracheary 
tissues  of  the  bundles  of  the  reticulate  cells  which  constitute  the 


CORDAITALES  AND  GINKGOALES  315 

opening  mechanism  of  the  microsporangia.  The  microsporophyll 
of  Ginkgo  accordingly  has  a  double  interest  from  the  evolutionary 
standpoint,  for  it  not  only  shows  the  centripetal  or  cryptogamic 
wood  more  clearly  than  it  is  exhibited  by  the  vegetative  leaves, 
but  at  the  same  time  manifests  its  transition  by  imperceptible 
gradations  into  the  mechanical  tissues  of  the  sporangium  wall. 
As  has  been  indicated  in  an  earlier  chapter,  Ginkgo  is  the  lowest 
type  in  which  the  dehiscence  of  the  microsporangium  no  longer 
depends  on  an  annulus  derived  from  the  epidermis  but  is  effected 
by  an  internal  mechanism  derived  from  the  old  centripetal  or 
cryptogamic  wood  of  the  fibrovascular  bundle. 

In  the  stalk  of  the  ovule  centripetal  elements  and  transfusion 
tissue  are  also  well  developed,  but  they  apparently  do  not  at  any 
time  penetrate  into  the  substance  of  the  megasporangium.  It  is 
not  unlikely  that  tracheary  tissues  of  a  transfusionary  nature  were 
formerly  present  in  the  megasporangial  structures  of  the  Gink- 
goales,  but  that  in  the  course  of  time  they  have  suffered  abortion. 
The  organization  of  a  number  of  seeds  of  Paleozoic  age  of  unascer- 
tained affinities  is  good  evidence  in  favor  of  the  probability  of  this 
view.  Moreover,  in  one  of  these,  Stephanos permum,  characterized 
by  a  tracheary  mantle  in  the  wall  of  the  nucellus  ending  in  the 
pollen  chamber,  pollen  grains  are  present,  winged  all  around  and 
strongly  resembling  those  of  Ginkgo.  It  is  accordingly  not  im- 
possible that  Stephanos  permum  was  the  seed  of  some  Paleozoic 
representative  of  the  Ginkgoales. 

The  importance  of  the  sole  surviving  and  monotypic  genus 
Ginkgo  from  the  standpoint  of  the  evolutionary  transition  from 
the  ancient  to  the  modern  gymnosperms  cannot  be  overestimated. 
It  constitutes  virtually  a  link  between  the  Archigymnospermae 
and  the  Metagymnospermae,  since  it  presents  to  so  large  a  degree 
the  characteristics  of  both.  Its  affinities  on  the  lower  side  are 
clearly  with  the  Cordaitales,  as  has  been  recognized  by  all  com- 
petent investigators  in  recent  years.  Its  relationship  with  the 
Abietineae  among  the  Coniferales  is  equally  well  indicated  by 
comparative  anatomical  and  paleobotanical  data,  as  will  be  shown 
in  the  following  chapter. 

The  indications  of  relationship  with  the  Cordaitales  are  pre- 
sented in  connection  with  the  organization  of  the  wood  in  primitive 


316  THE  ANATOMY  OF  WOODY  PLANTS 

organs  and  regions.  It  has  been  pointed  out  in  the  foregoing 
paragraphs  that,  although  centripetal  wood  of  the  cryptogamic 
type  is  represented  almost  exclusively  by  transfusion  tissues  in 
the  mature  vegetative  leaf  of  Ginkgo,  it  is  present  in  a  clearly 
recognizable  form  in  the  cotyledon,  in  the  microsporophylls,  and 
in  the  peduncle  of  the  ovuliferous  apparatus.  In  the  case  of  the 
microsporophyll  the  xylem,  and  more  particularly  the  vestigial 
centripetal  xylem  and  transfusion  tissue,  are  in  clear  relation  to 
the  reticulately  thickened  opening  mechanism  of  the  microspo- 
rangia.  This  feature  is  of  value,  not  only  as  indicating  the  filiation 
of  the  Ginkgoales  with  lower  groups,  but  also  as  indicating  the 
morphological  nature  of  the  arrangements  for  dehiscence  of  the 
spore  sac  in  the  seed  plants  above  the  Cycadales.  The  absence 
of  longitudinal  parenchyma  in  the  secondary  wood  is  another 
criterion  of  the  relationship  of  the  Ginkgoales  with  lower  groups, 
while  the  presence  of  tangential  pitting  in  the  terminal  region  of 
the  summer  wood  clearly  relates  the  group  with  modern  gymno- 
spermous  types.  The  radial  pitting  of  the  tracheids  and  associated 
structures  is  also  of  importance  as  indicating  the  phylogenetic 
position  of  the  genus.  As  has  been  shown  above,  the  radial  pits  of 
Ginkgo  are  opposite  in  the  mature  wood,  and  often  in  the  inter- 
vals between  them,  particularly  toward  the  ends  of  the  tracheids, 
have  transverse  bars  of  pectic  cellulose  imbedded  in  the  tracheary 
wall,  and  these  are  conveniently  designated  bars  of  Sanio. 
The  opposite  pitting  and  the  occurrence  of  bars  of  Sanio  are 
features  which  clearly  co-ordinate  the  wood  of  the  Ginkgoales 
with  that  of  the  higher  gymnosperms.  However,  in  the  primitive 
regions  and  organs  of  the  living  Ginkgo  we  find  both  the  pitting 
of  the  Cordaitales  and  the  absence  of  bars  of  Sanio  which  are 
universally  characteristic  of  the  older  gymnosperms  from  the 
Cycadales  downward.  It  seems  quite  obvious  that  Ginkgo  is  a 
genus  of  the  utmost  importance  from  the  standpoint  of  evolution- 
ary anatomy,  since  it  summarizes  in  such  a  remarkable  manner 
the  anatomical  characteristics  of  both  Archigymnospermae  and 
Metagymnospermae.  Its  significance  in  the  direction  indicated 
will  be  fully  realized  only  after  the  next  tribe,  the  Coniferales, 
have  been  anatomically  considered  in  the  following  chapter. 


CHAPTER  XXIV 
THE  METAGYMNOSPERMAE:    CONIFERALES 

As  has  been  indicated  at  an  earlier  stage,  the  gymnosperms 
are  somewhat  clearly  divisible  into  two  large  groups:  the  Archi- 
gymnospermae,  which  are  often  fernlike  in  habit  and  always 
cryptogamic  in  the  anatomical  organization  of  their  primary 
wood  and  in  their  mode  of  fertilization  by  antherozoids,  and  the 
Metagymnospermae,  which  present  no  external  resemblance  to 
the  members  of  the  fern  series,  and  in  which  the  centripetal  or 
cryptogamic  primary  xylem  has  given  place,  in  living  forms  at 
any  rate,  to  transfusion  tissue,  and  in  which,  also,  fertilization 
by  means  of  a  pollen  tube  is  a  universal  feature.  The  Coniferales 
are  the  largest  and  the  most  important  group  under  the  Meta- 
gymnospermae. Their  significance  from  the  evolutionary  stand- 
point can  scarcely  be  overestimated,  not  only  because  they  are 
more  abundantly  represented  in  the  living  plant  population  of 
the  earth  than  are  any  other  gymnosperms,  but  also  because 
they  are  copiously  preserved  as  fossils  as  far  back  as  the  Paleozoic 
age.  They  thus  supply  the  most  important  document  for  the 
inductive  study  of  general  principles  of  evolution  presented  by 
any  group  of  living  organisms,  vegetable  or  animal,  living  or 
extinct.  The  paleobotanical  and  anatomical  investigation  of 
the  Coniferales  has  greatly  changed  our  views  in  regard  to  their 
phylogenetic  sequence  in  recent  years.  The  older  students  of 
the  group  restricted  to  a  knowledge  of  living  forms  naturally 
assumed  that  the  conifers,  which  are  the  simplest  in  organization 
of  their  vegetative  and  reproductive  parts,  are  most  primitive. 
By  those  entertaining  this  view  the  Taxineae  are  considered  the 
most  ancient  conifers,  and  with  them  have  been  connected,  not 
only  the  Cordaitales,  but  also  the  living  genus  Ginkgo.  In  the 
most  recent  systematic  treatment  of  the  coniferous  tribe  as  a 
whole  we  find  this  attitude  maintained,  for  Ginkgo  and  Taxus 
are  regarded  as  closely  related.  It  is  needless  to  state  that  there 

317 


3i8  THE  ANATOMY  OF  WOODY  PLANTS 

is  nothing  in  common  between  the  anatomical  structures  of  vege- 
tative and  reproductive  parts  in  Ginkgo  and  the  Taxineae.  A 
later,  but  apparently  equally  erroneous,  tendency  is  to  interpret 
the  evolutionary  sequence  of  the  Coniferales  entirely  in  the  light 
of  the  data  derived  from  the  study  of  Paleozoic  gymnosperms. 
This  attitude  is  of  course  found  strongly  in  evidence  in  those 
countries  which  have  contributed  notably  to  the  elucidation  of 
the  organization  of  the  seed  plants  of  the  Paleozoic.  By  those 
who  are  affected  by  the  Paleozoic  bias  the  araucarian  subtribe  of 
the  Coniferales  is  considered  the  oldest  representative  of  the 
group.  There  are  those  again  who  attempt  to  reconcile  the 
taxinean  and  araucarian  hypotheses  of  derivation  by  the  assump- 
tion that  the  Araucariineae  have  come  from  the  Cordaitales, 
while  the  remaining  coniferous  subtribes  have  been  derived  from 
the  Taxineae.  The  hypothesis  of  a  lycopodineous  origin  of  the 
group  has  been  put  forward  at  various  times,  but  need  only  be 
mentioned  here,  as  it  has  few  advocates  and  does  not  appear  to 
derive  any  support  from  paleobotanical  or  anatomical  facts. 
The  views  in  regard  to  the  phylogeny  and  evolution  of  the  Co- 
niferales adopted  in  the  present  work  represent  an  attempt  to 
interpret  this  large  and  important  group  of  gymnosperms  in  the 
light  supplied  by  the  anatomy  of  Mesozoic  forms  as  compared 
with  representatives  of  the  group  still  living.  The  method  of 
treatment  adopted  will  be,  so  far  as  the  limits  of  space  in  an  ele- 
mentary textbook  permit,  purely  inductive.  As  has  been  em- 
phasized in  an  earlier  paragraph,  the  conifers,  on  account  of  their 
abundant  presence  in  the  floras  past  and  present,  supply  a  most 
valuable  document  for  the  interpretation  of  the  fundamental 
principles  of  evolution. 

Since  the  araucarian  conifers  are  quite  generally  regarded 
at  the  present  time  as  the  primitive  representatives  of  the  group, 
it  will  serve  a  useful  purpose  to  consider  these  first.  A  transverse 
section  of  the  wood  of  the  stem  in  this  subtribe  (Fig.  225)  generally 
reveals  the  presence  of  annual  rings,  unless  the  particular  species 
under  investigation  happens  to  be  of  lowland  tropical  origin.  The 
autumnal  tracheids  are  marked  by  tangential  pitting,  a  general 
feature  of  organization  of  the  more  modern  gymnosperms.  The 


CONIFERALES 


319 


wood  normally  shows  the  presence  of  only  radial  parenchyma,  lon- 
gitudinal storage  elements  being  absent.  The  longitudinal  radial 
section  of  the  wood 
(Fig.  226)  shows 
a  condition  of  pit- 
ting resembling  that 
found  in  the  Cordai- 
tales,  namely,  one 
which  is  alternating 
and  characterized 
by  the  absence  of 
the  bars  of  Sanio. 
The  absence  of  wood 
parenchyma  and  the 
alternating  char- 
acter of  the  pitting 
are  features  which  at 
first  sight  would 
seem  to  justify  the  FIG.  225.— Transverse  section  of  the  wood  of  Agathis 

assumption  of    a    ^tstraiis. 

close  degree  of  rela- 
tionship between 
the  araucarian  coni- 
fers and  the  Cordai- 
tales,  and  this  view 
of  their  affinities  has 
been  the  one  almost 
universally  adopted. 
Before  we  inquire  as 
to  its  validity  it  is 
necessary  to  exam- 
ine the  organization 
of  Mesozoic  repre- 
sentatives of  the 
group.  In  Fig.  227 

FIG.  226.— Longitudinal  section  of  the  wood  of  Ago-     1S    snown    m    trans- 
this  australis.  verse  section  the 


320 


THE  ANATOMY  OF  WOODY  PLANTS 


structure  of  a  Cretaceous  araucarian  wood  of  the  type  designated 
Araucarioxylon.    The  annual  rings  are  much  less  clearly  developed 

than  in  the  wood  of 
Agathis.  The  rays 
are  uniseriate,  as  in 
the  living  type,  but 
a  marked  contrast  is 
presented  by  the 
presence  of  wood 
parenchyma  in  the 
fossil.  The  longitu- 
dinal section  por- 
trayed in  Fig.  228 
shows  both  alter- 
nating pitting  and 
the  presence  of 
parenchyma.  Cer- 
tain of  the  tracheids 

FIG.  227. — Transverse  section  of  Araucarioxylon  from 
the  Cretaceous  of  the  eastern  United  States. 


are  also  filled  with 
dark  contents  de- 


rived as  an  exuda- 
tion from  the  rays. 
This  is  a  condition 
often  present  in  both 
living  and  extinct 
woods  of  araucarian 
affinities.  We  may 
now  turn  our  atten- 
tion to  another 
araucarian  type 
commonly  present 
in  the  Mesozoic — 
the  genus  Brachy- 
oxylon.  In  this  type 
the  wood  in  trans- 
verse section  shows 
annual  rings  and  the 


FIG.    228.— Longitudinal    section    of    the    same 
Araucarioxylon. 


CONIFERALES 


321 


absence  of  longitudinal  parenchyma.  In  longitudinal  radial  aspect 
it  manifests  a  kind  of  pitting  which  is  only  partially  araucarian, 
for  more  often  than 
not  the  pores  are 
separated  by  con- 
siderable intervals 
and  fail  to  alternate 
(Fig.  229).  It  is 
only  occasionally 
that  the  typical  ar- 
aucarian crowding 
and  alternation  are 
present.  More- 
over, in  woods  of 
the  Brachyoxylon 
type  wounding 
brings  about  the 

formation  of  trau- 

FIG.   229.— Longitudinal    section    of    the    wood    of 
Brachyoxylon. 


matic  resin  canals 
(Fig.  230)  such  as 
appear  after  injury  in 


certain  of 


FIG.  230. — Transverse  section  of  the  wood 
of  Brachyoxylon  formed  after  wounding. 


the  Abietineae   and  in  the 
genus  Sequoia. 

After  the  consideration 
of  the  conditions  in  the 
mature  wood  of  living  and 
extinct  representatives  of 
the  araucarian  conifers,  we 
may  now  turn  our  atten- 
tion to  the  organization  of 
the  xylem  in  the  conserv- 
ative organs  of  the  exist- 
ing araucarian  conifers. 
Fig.  231  illustrates  the 
structure  of  the  wood  in 
the  root  of  A  gat  his  australis 
in  a  region  not  very  remote 
from  the  primary  wood. 


322 


THE  ANATOMY  OF  WOODY  PLANTS 


It  is  evident  that,  in  the  secondary  xylem  of  the  root,  parenchyma, 
conspicuous  by  its  absence  in  the  mature  wood  of  the  stem,  is 
abundantly  present.  Not  only  is  this  the  case  with  the  root, 
but  the  same  situation  is  found  in  the  first  annual  ring  of  the 
vegetative  stem  and  also  very  strikingly  in  the  woody  axis  of  the 
ovuliferous  cone.  The  facts  here  mentioned  are  of  particular 
significance  when  correlated  with  the  organization  of  the  Cretace- 
ous Araucarioxylon 
shown  in  Fig.  227. 
Obviously  the 
parenchyma  pres- 
ent in  the  older 
type  of  araucarian 
wood  is  perpet- 
uated in  those  re- 
gions of  the  living 
form  which  we 
have  learned  in  an 
earlier  chapter  to 
regard  as  conserv- 
ative.  It  may 
accordingly  be 
logically  assumed 
that  woods  of  the 
type  of  the  living 
araucarian  conifers  formerly  possessed  longitudinal  parenchyma 
and  in  this  respect  are  at  variance  in  organization  with  the  ligne- 
ous structure  of  the  Paleozoic  Cordaitales.  This  conclusion  is 
reinforced  by  an  examination  of  the  effects  of  injury  in  the  living 
genera,  for  the  infliction  of  wounds  results  frequently  in  the  recall 
of  the  lost  parenchyma  even  in  the  adult  axis. 

We  may  now  pass  to  the  consideration  of  other  features  which 
are  supposed  to  indicate  a  close  degree  of  relationship  between 
the  Araucariineae  and  the  Cordaitales.  The  most  important 
of  these  are  the  crowded  pitting  and  the  absence  of  bars  of 
Sanio.  In  Fig.  232  is  shown  the  organization  of  the  wood  in 
the  stem  of  the  seedling  of  Agathis  australis  as  viewed  in  longitudinal 


FIG.  231. — Longitudinal  section  of  the  wood  of  the 
root  in  Agathis  australis. 


CONIFERALES 


323 


radial  section.  The  pits  are  neither  crowded  nor  alternating  as 
in  the  wood  of  the  adult.  An  examination  of  the  organization 
of  the  seedling  in  the  living  representatives  of  the  araucarian 
conifers  therefore  justifies  the  view  that  the  ancestral  forms  did 
not  possess  crowded  pitting.  Precisely  similar  conditions  are 
found  in  the  cone,  for  here  the  pits  in  the  tracheids  nearer  the 
primary  wood  lack  the  crowded  and  alternating  disposition  of 
the  mature  vegeta- 
tive wood  of  the 
genus.  But  a  still 
more  important 
feature  is  pre- 
sented by  the 
organization  of  the 
wood  of  the  ovu- 
liferous  cones  of 
the  living  A  gat  his 
and  Araucaria. 
Fig.  233  shows  a 
longitudinal  radial 
view  of  the  second- 
ary wood  of  Arau- 
caria Bidwillii  in 
the  vicinity  of  the 
protoxylem.  The 
pits  show  a  very  strong  tendency  to  opposition  in  arrangement, 
and  are  certainly  not  angular  by  mutual  contact,  as  is  often  the 
case  in  cordaitean  woods.  The  most  interesting  feature  shown 
by  the  figure,  however,  is  the  presence  of  bars  of  Sanio  such  as 
are  entirely  lacking  in  the  adult  vegetative  wood  of  existing  species 
of  the  araucarian  conifers.  As  a  consequence  of  the  situation 
revealed  in  the  conservative  reproductive  axis  of  the  araucarian 
conifers,  we  are  justified  in  assuming  that  the  absence  of  bars  of 
Sanio  and  the  presence  of  alternating  pitting  are  not  primitive 
features  of  the  organization  of  the  wood  of  the  subtribe,  and 
consequently  cannot  be  brought  into  court  to  prove  its  cor- 
daitean affinities.  The  evidence,  in  fact,  must  be  read  in  exactly 


FIG.  232. — Longitudinal  section  of  the  wood  of  the 
seedling  in  Agathis  australis. 


324 


THE  ANATOMY  OF  WOODY  PLANTS 


the  opposite  sense  from  that  in  the  Ginkgoales;  for,  as  has  been 
shown  in  the  preceding  chapter,  the  anatomical  facts  there  justify 
the  assumption  of  the  original  presence  of  cordaitean  structure, 
characterized  by  alternation  of  pitting  and  absence  of  bars  of 
Sanio.  In  the  case  of  the  Araucariineae,  on  the  contrary,  we 
must  assume  on  the  basis  of  the  structure  of  primitive  regions 
that  bars  of  Sanio  and  opposite  pitting  were  an  older  feature  of 


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FIG.  233. — Longitudinal  section  of  the  wood  of  the  cone  axis  in  Araucaria  Bid- 
willii  in  the  region  of  the  pith. 

organization  of  the  wood  and  that  they  are  as  a  consequence  not 
nearly  related  to  the  Cordai tales. 

It  becomes  clear,  when  we  consider  the  arguments  derived 
from  comparative  anatomical  data  and  those  furnished  by  the 
study  of  extinct  forms,  that  the  organization  of  the  mature  wood 
in  the  living  Araucariineae  cannot  be  accepted  as  sufficient  evidence 
of  their  relationship  with  the  Cordaitales.  The  persistent  foliar 
traces  of  the  two  living  araucarian  genera  have  been  regarded 
in  some  quarters  as  an  important  indication  of  their  primitive 
character.  Here  again  the  comparative  anatomical  situation 


CONIFERALES  325 

as  well  as  the  conditions  found  in  allied  fossil  conifers  do  not 
justify  the  conclusion  reached.  The  seedlings  of  both  Araucaria 
and  Agathis  show  the  earlier  leaf  traces  as  evanescent  structures 
which  cease  to  be  formed  by  the  cambium  after  the  leaves  to  which 
they  belonged  have  disappeared.  It  is  only  in  the  older  trunk 
that  the  formation  of  foliar  strands  is  perpetuated  for  many 
years,  amounting  even  to  centuries,  after  their  corresponding 
leaves  have  disappeared.  The  persistent  leaf  traces  which  con- 
stitute so  remarkable  a  feature  of  the  organization  of  the  mature 
trunk  of  the  existing  Araucariineae  cannot  therefore  be  regarded 
as  anything  but  a  bizarre  and  freakish  feature  which  has  no  evo- 
lutionary importance.  If  any  remaining  doubt  can  be  considered 
to  exist  on  the  subject,  it  is  set  aside  by  the  organization  of  the 
Mesozoic  araucarian  woods,  which  (with  the  exception  of  the 
type  known  as  Araucarioxylon) ,  are  distinguished  by  the  absence  of 
persistent  foliar  traces.  Comparative  anatomy  lends  very  little 
support  to  the  inference  of  cordaitean  affinities  for  the  araucarian 
conifers,  and  the  evidence  against  this  widely  cherished  view 
becomes  quite  overwhelming  when  the  anatomical  situation  in 
the  other  subtribes  of  the  Coniferales  is  considered.  The  further 
estimation  of  the  claims  of  the  Araucariineae  to  the  pre-eminence 
of  being  the  oldest  conifers  may  appropriately  be  delayed  until 
the  anatomical  features  of  other  important  subtribes  have  been 
examined. 

The  Abietineae  have  in  recent  years  made  progress  as  claimants 
to  the  primitive  position  among  Coniferales  and  to  that  of  nearest 
proximity  to  the  Cordaitales.  At  first  sight  the  extremely  com- 
plicated organization  of  both  vegetative  and  reproductive  struc- 
tures in  the  abietineous  conifers  appears  to  stand  in  the  way  of 
any  such  conclusion;  but  the  results  of  the  comparative  investiga- 
tion of  the  living  and  fossil  representatives  of  groups  which  have 
passed  the  zenith  of  their  development  has  taught  us  not  to  consider 
complexity  of  organization  as  necessarily  a  criterion  of  modernity. 
It  will  be  shown  in  subsequent  paragraphs  that  the  Abietineae 
are  in  a  very  strong  position  as  regards  primitiveness,  both  because 
they  are  apparently  related,  on  good  anatomical  evidence,  to  such 
ancient  groups  as  the  Cordaitales  and  Ginkgoales  and  because 


326 


THE  ANATOMY  OF  WOODY  PLANTS 


they  are  clearly  antecedent  to  the  mass  of  other  living  coniferous 
subtribes. 

The  mature  wood  of  Pinus,  illustrated  in  Fig.  234,  is  char- 
acterized in  transverse  section  by  the  presence  of  resin  canals  and 
by  rays  of  complex  organization.  The  longitudinal  structure  of 
the  wood,  as  is  shown  in  Fig.  235,  is  characterized  by  the  presence 

of  bars  of  Sanio.  The 
rays  are  of  complicated 
structure,  even  when  uni- 
seriate,  and  are  composed 
of  central  storage  cells 
and  marginal  elements 
resembling  tracheids.  If 
a  primitive  region  such  as 
that  provided  by  the 
wood  of  the  cone  axis  or 
root  be  investigated,  it 
becomes  clear  that  the 
bars  of  Sanio  are  not  an 
ancestral  feature  of 
organization  of  the  wood, 

since  they  are  absent  in 
FIG.  234. — Longitudinal  view  of  the  wood  in        .  .  ... 

Pinus  resinosa.  the    lnner    ^S1011    °f    the 

wood  of  the  cone  and  are 

also  often  lacking  in  the  tracheary  elements  of  the  root,  especially 
in  proximity  to  the  primary  wood.  This  situation  is  portrayed 
for  the  cone  of  the  Italian  nut  pine  (Pinus  pinea)  in  Fig.  236. 
It  is  obvious  that  the  tracheids  retain  for  some  time  the  spiral 
markings  of  the  primitive  region.  The  walls  of  the  tracheary 
elements  show  not  the  slightest  indication  of  the  presence  of  bars 
of  Sanio  until  a  region  remote  from  the  pith  has  been  reached. 
The  rays  also  are  without  the  marginal  tracheids  which  manifest 
themselves  at  an  early  stage  in  the  organization  of  the  wood  of 
the  vegetative  branches.  Clearly,  so  far  as  the  structure  of  the 
wood  in  the  reproductive  axis  is  concerned,  the  Abietineae  as 
represented  by  Pinus  are  derived  from  ancestors  possessing  the 
structure  of  the  wood  of  the  Cordaitales.  Another  interesting 


CONIFERALES  327 

feature  of  resemblance  to  cordaitean  forms  is  the  absence  of  a 
torus  in  the  membranes  of  the  bordered  pits  of  the  tiacheids  lying 
nearer  to  the  primary  wood.  The  situation  in  this  respect  is  the 
exact  opposite  of  that  found  in  the  case  of  the  araucarian  conifers, 


FIG.  235. — Highly  magnified  view  of  the  tracheids  of  a  species  of  pine,  showing 
the  bars  of  Sanio  (after  Gerry). 

in  which  the  torus  is  sometimes  present  in  the  region  near  the 
primary  xylem  of  the  wood  of  the  cone  axis  and  is  entirely  absent 
elsewhere.  The  organization  of  the  wood  alone  in  primitive 
organs  and  regions  justifies  the  conclusion  of  a  filiation  between 
the  Abietineae  and  the  Cordaitales  rather  than  between  the 
Araucariineae  and  the  Cordaitales. 


328 


THE  ANATOMY  OF  WOODY  PLANTS 


We  may  now  turn  to  the  brief  consideration  of  evidence  derived 
from  the  organization  of  the  wood  of  fossil  forms  and  bearing  on 
the  respective  antiquity  of  the  Abietineae  and  the  Araucariineae. 
The  investigations  of  recent  years  have  brought  to  light  in  the 
Jurassic  and  Cretaceous  numerous  coniferous  woods  which  to  a 
large  degree  possess  characteristics  intermediate  between  those 


S^c 


© 
fe 


© 


© 


w 


FIG.  236. — Transitional  region  from  the  xylem  of  the  cone  of  .Piwws  pinea 

of  the  Abietineae  and  Araucariineae.  The  conclusion  naturally 
follows  that  the  two  subtribes  were  less  remote  from  one  another 
in  Mesozoic  time  than  they  are  in  the  present  epoch.  The  question 
of  interpretation  is  strongly  debated  in  the  case  of  these  woods. 
The  mass  of  paleobotanists,  obsessed  by  the  araucarian  hypothesis 
of  the  derivation  of  the  Coniferales  from  their  cordaitean  ancestors 
and  little  concerned  with  the  fundamental  principles  of  comparative 


CONIFERALES 


329 


anatomy,  have  assumed  that  the  transitional  woods  in  question 
are  those  of  Abietineae  which  are  losing  their  primitive  araucarian 
characters.  A  fatal  objection  to  this  point  of  view,  however,  is 
the  fact  that  none  of  these  transitional  woods  shows  the  presence 
of  bars  of  Sanio.  In  other  words,  they  must  clearly  be  diagnosed 
as  belonging  to  the  araucarian  side  on  the  basis  of  the  most  reliable 
of  diagnostics  of  coniferous  woods — bars  of  Sanio.  Attempts 
in  the  direction  of  proving 
the  woods  in  question 
abietineous  rather  than 
araucarian  have  chiefly 
taken  the  form  of  discus- 
sions as  to  the  value  of  ray 
structure  in  the  diagnosis 
of  coniferous  woods.  It  is 
beyond  the  range  of  the 
present  volume  to  discuss 
details  of  the  organization 
of  the  radial  structures  in 
the  Coniferales;  but  it  may 
be  stated  in  a  general  way 


FIG.  237. — Wood  of  Brachyoxylon  formed 
after  wounding. 


and  on  the  basis  of  com- 
parative anatomy  that  no 
feature  is  more  subject  to  variability  within  the  limits  of  a  single 
subtribe  and  hence  is  less  available  for  comprehensive  conclusions 
in  regard  to  evolutionary  sequence.  A  final  argument  against  the 
araucarian  descent  of  the  Coniferales  from  the  Cordaitales  is 
supplied  by  the  extremely  abundant  Mesozoic  araucarian  type 
of  wood  known  as  Brachyoxylon.  In  wounded  specimens  of  wood 
of  this  genus  traumatic  resin  canals  are  formed  (Fig.  237), 
resembling  those  of  the  normal  wood  of  the  pinelike  conifers. 
The  occurrence  of  resin  canals  as  a  consequence  of  injury  in 
Brachyoxylon,  in  view  of  the  fact  that  this  genus  is  admitted 
by  competent  paleobotanists  to  be  of  unquestionable  araucarian 
affinities,  is  of  great  significance.  This  being  the  case,  we  are 
justified  in  interpreting  the  canals  formed  after  wounding  as  a 
reversionary  phenomenon,  indicating  relationship  to  the  pinelike 


33° 


THE  ANATOMY  OF  WOODY  PLANTS 


Abietineae.  This  interpretation  of  the  situation  is  vindicated 
by  the  recent  discovery  of  normal  resin  canals  in  the  wood  of 
the  axis  of  the  ovuliferous  cone  of  a  Javanese  species  of  Agathis, 
A.  Bidwillii  (Fig.  238). 

Having  discussed,  so  far  as  the  limits  of  the  present  volume 
permit,  the  organization  of  the  wood  in  conservative  axes  and  in 
fossil  forms,  we  must  now  turn  to  the  discussion  of  that  extremely 
important  organ,  the  leaf.  It  has  been  made  clear  in  an  earlier 

chapter  that  the  foliar 
organ  of  Pinus  is  char- 
acterized by  the  remark- 
able structure  of  its 
fibro vascular  tissues.  In 
the  genus  under  discussion 
and  to  a  less  extent  in 
allied  genera  the  foliar 
conducting  strand  is  sur- 
rounded by  a  cordon  of 
transfusion  tissue.  The 
situation  in  this  respect 
may  be  clearly  ascertained 
by  reference  to  Fig.  239. 
The  transfusion  elements 
are  distinguished  by  the 
absence  of  protoplasmic  contents  and  by  the  occurrence  of 
bordered  pits  in  their  walls.  It  is  obvious  that  the  tissues  of  this 
nature  become  joined  with  the  xylem  of  the  foliar  bundles  on  its 
flanks.  The  transfusion  tissue  in  modern  representatives  of  the 
genus  Pinus  is  not  a  continuous  mass  of  tracheary  cells,  but  has 
interspersed  throughout  its  substance  a  considerable  number  of 
living  cells  provided  with  protoplasm  and  a  nucleus.  The  investi- 
gation of  the  Cretaceous  deposits  at  Kreischerville,  Staten  Island, 
has  provided  us  with  extremely  valuable  data  for  the  determina- 
tion of  the  organization  of  the  leaf  in  Pinus  and  allied  forms  in 
the  later  Mesozoic.  In  some  of  the  numerous  species  of  Pinus 
which  flourished  in  the  American  Cretaceous,  transfusion  tissue 
was  present  in  large  amount  and  contained  little  or  no  admixture 


FIG.  238. — Resin  canal  in  the  trace  of  the 
seed  scale  of  Agathis  Bidwillii. 


CONIFERALES  331 

of  parenchymatous  elements.  In  still  other  species  the  short 
tracheary  elements  ordinarily  called  transfusion  cells  were  much 
less  well  developed.  It  is  the  remarkable  genus  Prepinus,  however, 
which  provides  the  most  important  evidence  for  estimating  the 
bearing  of  the  anatomical  organization  of  the  leaf  on  the  problem 
of  evolution  of  the  genus  Pinus  in  particular  and  that  of  the 
Abietineae  in  general.  In  Fig.  240  is  shown  the  transverse  section 


FIG.  239. — Leaf  bundle  of  Pinus  Strobus 

of  the  leaf  in  P.  statenensis.  The  outline  is  angular  because  of 
mutual  contact  with  other  and  surrounding  leaves  of  the  fascicle. 
In  Prepinus  the  growing  point  of  the  short-shoots  persisted  as  it 
does  in  the  living  Ginkgo,  and  the  fascicular  leaves,  instead  of  being 
few  and  definite  in  their  number,  were  indefinitely  numerous. 
It  is  interesting  to  note  in  this  connection,  however,  that,  although 
in  the  true  Pinus  of  the  Cretaceous  the  number  of  leaves  in  the 
fascicle  was  few  and  fixed  as  in  modern  forms,  nevertheless  the 
growing  point  of  the  short-shoot  persisted  indefinitely  and  did 
not  disappear  at  an  early  stage,  as  in  the  living  representative  of 


332  THE  ANATOMY  OF  WOODY  PLANTS 

the  genus.  Within  the  angular  outline  of  the  leaf  in  Prepinus 
is  seen,  beneath  the  epidermis,  the  ribbed  hypodermal  tissues, 
recalling  those  of  the  older  gymnosperms.  The  cortical  region 
of  the  leaf  terminates  in  a  not  very  clearly  marked  endodermis 
which  doubtless  owes  its  loss  of  denniteness  to  the  changes  resulting 
from  fossilization.  Within  the  endodermal  boundary  lies  the 
entirely  tracheary  and  strongly  pitted  transfusion  tissue  of  the 


FIG.  240. — Leaf  of  Prepinus  slatenensis 

leaf.  The  elements  of  this  category  are  differentiated  into  two 
zones — an  outer  one  composed  of  short,  broad,  pitted  elements, 
and  an  inner  one  consisting  of  thick-walled  cells  of  narrow  lumen. 
The  former  are  known  as  the  outer  transfusion  sheath  and  the 
latter  as  the  inner  transfusion  sheath.  Both  structures  have 
their  counterpart  in  the  leaf  of  certain  of  the  Cordaitales.  The 
double  transfusion  sheath  was  also  frequently  present  in  the  foliar 
organs  of  true  pines  of  the  American  Cretaceous.  Another  most 
interesting  feature  of  the  organization  of  Prepinus  was  the  struc- 
ture of  the  xylem.  As  may  be  seen  from  Fig.  241,  the  wood 
presents  two  regions,  an  upper  and  a  lower.  In  the  former  the 


CONIFERALES 


333 


elements  are  arranged  in  radial  rows  and  usually  increase  in  size 
toward  the  upper  surface  of  the  leaf.  In  the  xylem  directed  down- 
ward are  seen  indications  of  wood  rays,  and  the  inspection  of  the 
longitudinal  aspect  reveals  the  fact  that  it  is  made  up  largely  of 
tracheids  with  bordered  pits.  This  is  the  centrifugal  xylem  and 
corresponds  to  the  mass  of  wood  in  the  foliar  trace  of  the  fascicular 


FIG.  241. — Portion  of  leaf  of  Prepinus  statenensis 

leaves  of  living  pines.  The  upwardly  developing  wood  is  the 
cryptogamic  xylem  and  confirms  the  conclusion  as  to  the  affinity 
of  Prepinus,  already  suggested  by  the  organization  of  its  trans- 
fusion sheath,  namely,  that  the  genus  is  allied  to  the  Cordaitales. 
It  is  thus  apparent  that  the  details  of  organization  of  the  leaf  in 
Prepinus,  which  in  turn  is  clearly  the  ancestor  of  Pinus,  justify 
an  attribution  of  cordaitean  ancestry  to  the  Abietineae.  This 
conclusion  as  to  relationship  is  supported  by  the  primitively  cor- 
daitean character  of  the  pitting  which  so  strikingly  indicates  a 


334 


THE  ANATOMY  OF  WOODY  PLANTS 


relationship  of  the  Abietineae  rather  than  the  Araucariineae  with 
the  Paleozoic  gymnosperms  known  as  Cordaitales. 

Not  only  do  the  Abietineae  as  a  result  of  their  anatomical 
organization  and  paleobotanical  history  present  a  strong  claim 
to  direct  relationship  with  the  older  gymnosperms,  but  they 
supply  equally  compelling  evidence  that  they  are  ancestral  to 
other  prominent  coniferous  subtribes.  It  will  be  well  in  this 

connection  to 
begin  with  the  in- 
ternal situation  in 
the  abietineous 
subtribe.  It  is 
readily  subdivided 
into  two  series  on 
the  basis  of  ana- 
tomical structure— 
the  Pineae  and  the 
Abieteae.  The 
former  are  char- 
acterized by  the 
possession  of  well- 
developed  resin 

canals  in  the  wood 
jn  both  vertical 

and  horizontal 
planes.  In  contrast  to  these,  in  the  second  series  the  h'gneous  resin 
canals  of  the  secondary  wood  are  notably  absent.  It  is  only  in 
regions  recognized  as  conservative  that  they  make  their  appear- 
ance —  in  the  primary  structures  of  the  xylem  of  the  root  (Fig.  242), 
in  the  secondary  wood  of  the  axis  of  the  ovuliferous  cone,  and  some- 
times in  the  first  annual  ring  of  the  vegetative  branches.  Further, 
resin  canals  are  found  in  the  wood  of  the  Abieteae  as  a  result  of  in- 
jury. Both  comparative  anatomical  and  experimental  evidence, 
as  a  consequence,  vouch  for  the  derivation  of  the  Abieteae  from 
ancestral  forms  possessing  well-developed  ligneous  resin  canals.  It 
is  of  significance  to  note  in  this  connection  that  the  genus  Cedrus, 
for  the  great  antiquity  of  which  the  geological  record  supplies  clear 


FIG.   242.—  Transverse  section  of  the  root  of  Abies 
balsamea,  showing  the  presence  of  a  resin  canal  in  the 


CONIFERALES 


335 


testimony,  not  only  from  American,  but  also  from  European  de- 
posits, is  strikingly  distinguished  by  the  fact  that  it  produces 
both  horizontal  and  vertical  resin  canals  resulting  from  injury 
(Fig.  243).  This  condition  is  in  contrast  to  that  manifested  by  the 
other  genera  of  the  Abieteae,  in  which  only  vertical  resin  canals 
make  their  appearance  in  the  secondary  wood  after  wounding.  It 
is  now  generally  admitted  by  competent  anatomists  that  there  is 
strong  evidence  for  the 
derivation  of  the  Abieteae 
from  the  Pineae  as  a  re- 
sult of  reductionaryr 
modification.  This  con- 
clusion is  reached,  not 
only  on  the  testimony 
supplied  by  the  resin 
canals  as  described  above, 
but  also  from  the  com-! 
parative  anatomical 
consideration  of  the 
organization  of  the  rays 
and  the  parenchyma  of 


FIG.  243. — Transverse  section  of  the  wood  of 
Cedrus  deodara  formed  after  injury,  showing 
reversionary  appearance  of  resin  canals  in  both 
vertical  and  horizontal  planes. 


the  secondary  wood.     It 
is  apparent  in  regard  to 
these  particular  struc- 
tures that  the  Pineae  are ' 
more  primitive  than  are  the  Abieteae.    The  ray  of  the  Abieteae  , 
is  often  characterized  by  the  loss  of  the  marginal  tracheids  so  dis-  ,' 
tinctively  developed   in   the    radial    parenchymatous   strands   of 
Pinus  and  its  living  allies. 

The  internal  conditions  in  the  Pineae  may  now  claim  our 
attention.  Here  we  find  a  striking  separation  between  Pinus  on 
the  one  hand  and  Picea,  Larix,  and  Pseudotsuga  on  the  other, 
resulting  from  a  consideration  of  the  lining  of  the  resin  canals 
in  the  wood.  In  the  first-named  genus  the  secretory  canals  are 
lined  by  thin-walled  parenchyma  which,  in  the  transformation 
of  heartwood  into  sapwood,  develops  processes  known  as  tyloses, 
which  more. or  less  completely  occlude  the  resin  canals.  In  the 


336 


THE  ANATOMY  OF  WOODY  PLANTS 


three  remaining  genera  of  the  Pineae  the  lining  of  the  secretory 
space  is  composed  mostly  of  thick-walled  more  or  less  lignified 
cells.  The  resin  canals  in  these  forms  do  not  accordingly  contain 
well-developed  tyloses  in  the  heartwood.  Another  important 
distinction  between  Pinus  and  allied  genera  is  the  complete  absence 
of  wood  parenchyma  in  the  former.  It  has  been  shown  in  an 
earlier  chapter  that  longitudinal  storage  parenchyma  is  formed 
in  the  secondary  wood  as  the  result  of  the  modification  of  elements 


FIG.  244. — Wood  of  Picea  canadensis,  showing  terminal  parenchyma 

destined  to  be  tracheids.  In  Picea,  Larix,  and  Pseudotsuga  storage 
parenchyma  is  present,  but  at  the  end  of  the  annual  ring  only 
(Fig.  244).  In  this  position,  particularly  in  the  case  of  the  root,  it 
manifests  convincing  evidence  as  to  its  derivation  by  the  occurrence 
of  transitional  stages  between  merely  septate  tracheids  and  rows  of 
parenchymatous  elements,  resembling  in  their  general  configuration 
tracheary  elements.  In  Pinus,  therefore,  there  is  no  true  paren- 
chyma of  the  wood,  since  such  storage  cells  are  found  only  in  the 
three  other  genera  of  the  Pineae.  Where  wood  parenchyma  is 
present,  moreover,  it  is  confined  to  the  end  of  the  annual  ring  and 
is  clearly  in  a  condition  of  derivation  from  tracheids,  a  state  found 
normally  in  no  other  living  representative  of  the  Coniferales.  It 


CONIFERALES  337 

has  been  demonstrated  in  earlier  pages  that  Paleozoic  gymno- 
sperms  are  characterized  by  the  complete  absence  of  parenchyma- 
tous  elements  in  the  wood  and  at  the  same  time  by  the  general 
absence  of  annual  rings  in  the  stem.  Pinus,  as  regards  the  organ- 
ization of  the  storage  devices  of  the  wood,  is  therefore  clearly 
allied  with  Paleozoic  types  such  as  the  Cordaitales. 

There  are  other  conditions,  however,  which  indicate  for  Pinus 
a  primitive  position  among  the  Abietineae.  First  of  all  there  is 
the  possession  of  short-shoots.  Pinus  in  this  feature  of  organiza- 
tion presents  a  marked  resemblance  to  the  Ginkgoales,  which 


FIG.  245. — Microspores  of  Ginkgo  and  Abies 

also  bear  their  foliar  organs  on  special  spurs  or  short-shoots. 
Nor  is  the  common  possession  of  short-shoots  unparalleled  by  other 
significant  characteristics.  Ginkgo  and  the  Abietineae  strongly 
resemble  one  another  in  the  possession  of  bisporangiate  sporo- 
phylls.  In  the  two  groups  there  are  two  microsporangia  and 
two  megasporangia  or  seeds  on  the  reproductive  foliar  organs. 
The  view  sometimes  advanced  that  the  ovuliferous  scales  in  the 
Abietineae  consist  of  a  fused  pair  of  foliar  structures  has  apparently 
no  evidence  in  its  favor.  It  is  as  clearly  a  single  leaf  as  is  the 
microsporophyll.  The  microspores  in  the  Abietineae  and  in  the 
Ginkgoales  also  present  striking  points  of  resemblance  which 
have  only  recently  been  completely  realized.  In  the  monotypic 
Ginkgo  the  pollen  is  winged  as  in  the  more  primitive  Abietineae 
and  resembles  in  its  internal  organization  the  structures  found 
in  the  microspores  of  that  subtribe  of  conifers.  Fig.  245  illustrates 
the  numerous  features  of  internal  and  external  resemblance  between 


338  THE  ANATOMY  OF  WOODY  PLANTS 

the  pollen  of  the  Abietineae  and  that  of  the  Ginkgoales.  Nor 
is  the  similarity  confined  to  the  structure  of  the  microspores. 
It  has  been  pointed  out  in  an  earlier  chapter  that  the  organ- 
ization of  the  wall  of  the  sporangium  in  relation  to  the  opening 
mechanism  and  to  its  derivation  from  the  fibrovascular  structures 
is  practically  identical  in  the  Ginkgoales  and  Abietineae.  Finally, 
the  organization  of  the  tracheids  of  the  wood  is  similar  in 
the  case  of  the  two  groups  under  consideration.  Pinus  seems 
beyond  question,  by  the  possession  of  short-shoots,  the  number 
and  organization  of  its  microsporangia  and  megasporangia,  as 
well  as  by  the  structure  of  its  microspores,  the  general  organization 
of  the  wood,  and,  finally,  by  the  absence  of  true  wood  parenchyma, 
clearly  allied  to  the  sole  surviving  genus,  Ginkgo. 

Pinus,  lastly,  presents  a  very  strong  claim  to  primitiveness 
among  the  Coniferales  by  reason  of  the  general  presence  of  short- 
shoots,  such  as  are  usually  regarded  as  the  prototypes  of  the 
ovuliferous  scales  of  the  female  cone  of  the  Coniferales  as  a  whole. 
It  follows,  on  the  general  principles  of  anatomy  laid  down  in  an 
earlier  chapter,  that  the  genus  which  still  shows  vegetatively  the 
structures  known  as  short-shoots  is  in  an  excellent  position 
to  claim  a  primitive  position  among  the  Coniferales.  The  absence 
of  short-shoots  in  the  seedling  of  Pinus  is  obviously  no  ground  for 
an  argument  of  any  significance  against  the  primitive  presence  of 
short-shoots  in  the  genus.  Negative  evidence  furnished  by  seed- 
lings is  of  no  value,  since  only  positive  testimony  in  connection 
with  the  hypothesis  of  recapitulation  can  be  accepted  as  valid 
in  evolutionary  argument.  We  may  therefore  assume  that  the 
presence  of  short-shoots  in  Pinus  and  Ginkgo,  as  well  as  many 
other  features  of  resemblance  between  the  two  genera,  is  an  unmis- 
takable indication  of  affinity.  Further,  since  short-shoots  are 
very  generally  assumed  to  have  been  the  prototype  of  the  ovulif- 
erous scale  in  the  female  cones  of  the  Coniferales  throughout,  we 
may  infer  that  the  coniferous  genus  which  has  manifested  these 
structures  as  a  normal  vegetative  feature  from  remote  geological 
times  must  be  a  very  ancient  representative  of  the  Coniferales. 

But  we  are  not  by  any  means  limited  to  a  consideration  of 
the  general  organization  of  the  female  cone  in  inferences  regarding 


CONIFERALES 


339 


the  relationship  of  Pinus  to  the  other  Coniferales.  An  excellent 
illustration  of  the  value  of  anatomical  evidence  in  the  case  of 
this  problem  is  furnished  by  the  interesting  taxodineous  genus 
Sequoia.  The  Taxodineae  as  well  as  the  nearly  allied  Cupres- 
sineae  are  characterized  anatomically  by  the  organization  of  the 
female  cone  and  the  structure  of  the  wood.  The  scales  of  the 
cone  are  superficially  single,  but  in  section  they  show  the  presence 
of  a  double  series  of  oppositely  orientated  fibrovascular  bundles, 
thus  indicating  the  origin  of  the  seed  scales  from  the  externally 


FIG.  246. — Transverse  section  of  the  cone  scale  of  Sequoia  gigantea,  showing  a 
double  system  of  bundles  with  opposite  orientation. 

double  structures  of  the  ovuliferous  cone  of  the  Abietineae 
(Fig.  246).  In  the  organization  of  their  wood  the  Taxodineae  differ 
from  the  Abietineae  in  the  absence  of  resin  canals.  There  is,  how- 
ever, a  resiniferous  secretion  produced  by  scattered  parenchymatous 
elements  of  the  wood.  In  the  structure  of  the  radial  parenchyma 
a  condition  of  simplicity  contrasting  with  that  found  in  the  Abie- 
tineae is  manifested,  for  the  marginal  tracheids  of  the  rays  of  the 
Abietineae  are  conspicuously  absent  in  the  normal  wood  of  the 
Taxodineae  in  general  and  of  Sequoia  in  particular.  If  we  con- 
sider the  genus  Sequoia  in  the  light  of  the  canons  of  anatomy 
formulated  above,  very  interesting  results  are  reached.  First,  if  a 
transverse  section  of  the  axis  of  the  cone  or  of  the  ovuliferous 
scale  of  Sequoia  gigantea  be  examined,  resin  canals  reveal  them- 
selves in  the  wood  in  proximity  to  the  primary  xylem  (Fig.  247). 


340 


THE  ANATOMY  OF  WOODY  PLANTS 


Further,  an  investigation  of  the  first  annual  ring  of  the  stem  in 
trees  which  have  attained  such  vigor  of  development  as  to  pro- 
duce seed  often 
shows  the  pres- 
ence of  resin  canals 
such  as  are  not 
normally  found  in 
the  subsequent  an- 
nual increments  of 
growth  (Fig.  248). 
Finally,  the  fibro- 
vascular  strand  of 
the  leaf  frequently 
contains  in  the 
region  of  the  xylem 
a  single  resin  canal. 
The  occurrence  of 
ligneous  resin 
canals  in  the  van- 
primitive  re- 


FIG.  247. — Transverse  section  of  reproductive  axis 
of  Sequoia  gigantea,  showing  resin  canals  in  the  wood. 


OUS 


gions  indicated 
above  is  good  evi- 
dence of  the  origi- 
nal presence  of 
such  structures  in 
the  woody  tissues 
of  Sequoia.  Addi- 
tional information 
on  this  subject  is 
furnished  by  the 
wound  reactions  of 
the  genus.  In 
either  of  the  two 
species,  S.  gigantea 
or  S.  sempervirens, 
the  infliction  of 
wounds  may  be 


FIG.  248. — Twig  of  Sequoia  gigantea,  showing  pres- 
ence of  resin  canals  in  the  first  annual  ring. 


CONIFERALES  341 

followed  by  the  appearance  of  traumatic  or  wound  resin  canals  as 
a  reversionary  feature.  In  S.  sempervirens,  the  redwood,  this  is  the 
only  mode  of  occurrence  of  these  structures.  It  may  be  added  in 
this  connection  that  the  normal  seedling  of  Sequoia  (either  species) 
shows  no  resin  canals  in  the  wood.  On  the  fallacious  logic  that 
structures  absent  in  the  seedling  are  not  ancestral  it  could  be 
argued  that  resin  canals  in  the  secondary  wood  are  not  an  ances- 
tral feature  of  the  genus  Sequoia.  This  genus  is  the  only  one  in 
any  coniferous  subtribe,  other  than  the  Abietineae,  characterized 
in  living  species  by  the  formation  of  traumatic  resin  canals.  In 
Sequoia  a  sound  inference  based  on  the  principle  of  conservative 
organs  is  that  its  ancestral  forms  were  provided  with  resin  canals 
throughout  their  structure. 

Although  only  the  genus  considered  in  the  foregoing  paragraph 
presents  the  abietineous  feature  of  resin  canals  in  the  secondary 
wood  as  a  result  of  injury,  many  genera  of  both  taxodineous  and 
cupressineous  affinities  revert  to  the  abietineous  type  of  ray  as 
a  response  to  wounding.  This  is  notably  the  situation  in  Sequoia 
and  is  illustrated  in  Fig.  53.  In  another  figure  (Fig.  52)  the  occur- 
rence of  marginal  tracheids  is  indicated  for  the  cupressineous  species 
Chamaecyparis  nootkatensis.  The  investigations  of  Miss  Holden 
have  made  it  clear  that  the  presence  of  traumatically  recalled 
ray-tracheids  is  a  common  feature  of  the  Cupressineae  and  the 
Taxodineae.  In  neither  of  these  tribes  are  such  structures  known 
to  occur  normally  in  conservative  organs;  hence  their  former 
presence  is  revealed  only  by  reversionary  phenomena.  It  is 
worth  while  to  note  that  the  recurrence  of  marginal  tracheids 
as  the  result  of  injury  is  usually  exemplified,  not  in  the  immediate 
region  of  the  wound  where  hypertrophy  alone  prevails,  but  in  a 
region  of  the  stem  more  or  less  remote  from  the  actual  injury. 
This  situation  is  of  interest  because  it  is  paralleled  by  conditions 
found  in  connection  with  certain  other  wound  reactions,  notably 
those  presented  in  the  case  of  the  rays  in  certain  angiosperms.  It 
is  obvious  from  what  has  been  stated  above  that  ray-tracheids, 
although  of  much  wider  occurrence  among  the  Cupressineae  and 
Taxodineae,  probably  appear  only  as  the  consequence  of  experi- 
mental conditions  and  are  no  longer  a  normal  feature  of  structure. 


342  THE  ANATOMY  OF  WOODY  PLANTS 

Normal  and  traumatic  resin  canals  and  traumatic  ray-tracheids 
are  abietineous  structures  occurring  in  certain  Taxodineae  and 
Cupressineae,  facts  which,  in  accordance  with  the  general  prin- 
ciples of  comparative  anatomy  already  elucidated,  may  be  regarded 
as  indicating  the  abietineous  origin  of  the  two  coniferous  subtribes 
in  which  they  occur.  This  conclusion  is  now  somewhat  generally 
accepted  by  those  whose  anatomical  knowledge  of  the  conifers 
makes  their  opinion  of  weight. 

Before  we  take  leave  of  the  two  subtribes  considered  in  the 
foregoing  paragraphs  it  will  be  well  to  direct  attention  to  the 
distribution  and  origin  of  wood  parenchyma  in  woods  of  this 
type.  The  secondary  xylem  is  characterized  by  the  presence 
of  usually  abundant  wood  parenchyma,  not  confined  to  the  end 
of  the  annual  rings,  but  scattered  throughout.  The  parenchyma- 
tous  elements  secrete  a  generally  highly  antiseptic  essential  oil. 
As  a  result  of  the  presence  of  essential  oils,  and  sometimes  also 
by  the  infiltration  of  the  tracheary  walls  with  tannin,  woods  of 
taxodineous  and  cupressineous  origin  are  frequently  resistant  to 
decay.  The  oil-secreting  cells  in  the  subtribes  under  discussion 
do  not  under  normal  conditions  betray  their  derivation  from 
tracheids  except  by  the  fact  that  they  are  grouped  in  series  which 
have  the  fusiform  shape  of  tracheary  elements.  In  injured  woods 
it  is  often  possible  to  observe  transitions  between  septate  tracheids 
and  rows  of  parenchymatous  elements.  It  may  accordingly  be 
assumed,  independently  of  the  evidence  of  abietineous  afhnities 
supplied  in  the  previous  paragraphs,  that  the  storage  elements 
in  the  woods  of  the  Cupressineae  and  Taxodineae  are  of  tracheary 
origin.  Another  feature  must  be  considered  in  this  connection. 
In  the  minds  of  those  who  regard  the  Coniferales  as  an  ascending 
series  and  not  one  of  reduction,  the  resin  canals  which  characterize 
the  wood  structure  of  the  older  Abietineae  owe  their  origin  to 
the  clustering  of  the  resin  cells  of  the  cupressineous  or  taxodineous 
type.  This  view  of  the  origin  of  secretory  canals  in  the  coniferous 
series  has  been  particularly  emphasized  by  Penhallow.  It  meets 
with  numerous  difficulties,  the  chief  of  which  is  that  the  cells 
which  surround  the  resin  canals  are  not  resin  cells.  The  latter 
possess  dark-brown  contents  and  produce  their  secretion  in  an 


CONIFERALES 


343 


intracellular  manner.  The  epithelial  cells  of  the  resin  canals  in 
the  abietineous  conifers,  on  the  contrary,  do  not  manifest  the 
dark-brown  (so-called)  "resin,"  but  pour  their  secretion  at  once 
into  the  secretory  space.  The  secretory  canals  found  in  the  case 
of  Sequoia  gigantea  are  surrounded  by  cells  which  correspond  only 
to  a  very  limited  extent  to  the  resin  cells  of  cupressineous.  woods. 
Most  of  the  secretory  elements  are  devoid  of  the  so-called  "resin" 
or  dark-brown  contents,  and 
those  containing  this  substance 
are  present  in  about  the  same 
proportion  near  the  canal  as 
they  are  in  the  structure  of  the 
adjacent  wood.  This  situation 
is  clearly  revealed  in  Figs.  249 
and  250. 

In  addition  to  the  Taxineae, 
Araucariineae,  Abietineae,  Cu- 
pressineae,  and  Taxodineae — 
sub  tribes  of  the  Coniferales 
which  have  been  discussed  to 
a  greater  or  less  extent  in  the 
earlier  paragraphs  of  the  pres- 
ent chapter — there  remain  the 
Podocarpineae,  a  group  which 
in  the  present  period  is  con- 
fined almost  entirely  to  the 
Southern  Hemisphere.  The 
podocarpineous  forms  are 

generally  regarded,  and  with  a  strong  degree  of  probability,  as 
somewhat  closely  allied  to  the  Taxineae  or  yews,  which  have  their 
main  distribution  in  the  Northern  Hemisphere.  They  are  char- 
acterized, however,  by  a  less  degree  of  simplification  in  their 
ovuliferous  cones  and  frequently  by  the  possession  of  winged 
pollen  of  the  abietineous  type,  produced  always  in  bisporangiate 
microsporophylls.  The  organization  of  the  female  cone  in  the 
Podocarpineae  as  a  whole,  and  particularly  in  the  genus  Podocarpus, 
recalls  by  the  presence  of  a  bract  and  a  subtending  ovuliferous 


FIG.  249. — Transverse  section  of  a 
traumatic  or  wound   resin  canal   of 

Sequoia  sempervirens. 


344 


THE  ANATOMY  OF  WOODY  PLANTS 


scale  the  conditions  found  in  the  double  scales  of  the  ovuliferous 
strobilus  of  the  Abietineae.  The  reproductive  features  conse- 
quently supply  some  evidence  for  the  association  of  the  podocarps 

with  the  abietineous  conifers. 
The  organization  of  the  wood  in 
the  group  under  consideration  is 
very  similar  to  that  found  in  the 
Cupressineae  and  Taxodineae. 
The  tracheids  possess  opposite 
pitting  and  bars  of  Sanio.  The 
parenchyma  is  abundant  and 
scattered  throughout  the  annual 
ring.  A  clear  difference  from  the 
Cupressineae  and  Taxodineae  is 
supplied,  however,  by  experi- 
mental evidence,  since  neither 
traumatic  resin  canals  nor  margi- 
nal ray-tracheids  have  been  found 
as  yet  in  any  of  the  genera  of  the 
Podocarpineae.  We  shall  prob- 
ably not  make  further  advance 
in  the  final  determination  of  the 
phylogenetic  or  evolutionary 
position  of  this  subtribe  of  the 
Coniferales  until  our  present  ex- 
tremely meager  knowledge  of  the 
fossil  conifers  of  the  Southern 
Hemisphere  has  been  notably  in- 
creased. It  seems  highly  prob- 
able, on  the  basis  of  the  organiza- 
tion of  the  scales  of  the  female 


FIG.  250. — Longitudinal  section 
of  a  traumatic  resin  canal  in  the  same 
species. 


cone  and  of  the  sporophylls  and 
spores  of  the  male  cone,  that  the  Podocarpineae  have  abietineous 
affinities.  Their  final  position  will  ultimately  be  determined  by  a 
better  knowledge  of  the  anatomy  of  extinct  forms. 

Of    the    coniferous    subtribes    enumerated    in    the    preceding 
pages  the  Taxineae  have  not  as  yet  been  considered  anatomically. 


CONIFERALES 


345 


The  reproductive  structures  in  this  group  are  extremely  simple, 
and  the  ovuliferous  apparatus  produces  in  maturity  a  single  ovule. 
In  the  genus  Cephalotaxus  of  the  Eastern  Hemisphere  an  ovuliferous 
strobilus  is  present  in  the  early  stage  of  development;  this  is 
composed  of  scales  each  producing  a  pair  of  ovules — a  condition 
comparable  with  that  found  in  the  Abietineae.  As  the  develop- 
ment of  the  seed  progresses  all  but  a  single  seminal  structure 
abort,  so  that  in  the 
end  no  vestige  of  the 
presence  of  a  true  fe- 
male cone  can  ordi- 
narily be  observed. 
In  accordance  with 
the  principle  of  re- 
capitulation, we  must 
regard  the  young  cone 
of  Cephalotaxus  as  in- 
dicating  the  more 
primitive  condition 
for  the  ovuliferous 
apparatus  in  the 
genus.  Taxus  pre- 
sents a  marked  con- 
trast to  that  genus  by 
the  fact  that  at  no 


FIG.  251. — Semidiagrammatic  view  of  the  longitudi- 
nal organization  of  the  wood  in  the  Taxineae. 


time  is  the  presence  of  a  female  or  ovuliferous  cone  indicated.  From 
the  first  there  is  but  a  single  ovule,  and  this  is  not  related  at  any  time 
to  a  visible  ovuliferous  scale.  The  organization  of  the  male  cones  in 
the  Taxineae  is  naturally  less  reduced  than  that  of  the  female,  in 
accordance  with  the  general  principle  of  the  conservatism  of  the 
microsporangial  apparatus  in  heterosporous  groups.  In  the  case  of 
the  Taxineae  the  reproductive  structures  furnish  convincing  evidence 
for  the  origin  of  the  subtribe  as  the  result  of  a  process  of  reduction. 
The  structure  of  the  wood  in  the  Taxineae  is  quite  characteristic. 
The  tracheids  are  marked  by  internal  spiral  bands  which  are  of 
late  origin  and  are  frequently  for  that  reason  designated  as  tertiary 
thickenings  (Fig .  2  5 1 ) .  Opposite  pitting  is  present  when  the  pores  are 


346 


THE  ANATOMY  OF  WOODY  PLANTS 


numerous,  and  bars  of  Sanio  are  clearly  developed.  The  state- 
ment is  often  made  that  the  ligneous  structure  of  the  Taxineae 
is  free  from  parenchymatous  elements  (Fig.  252).  This  is  certainly 
true  of  the  mature  wood  of  the  stem  in  both  Taxus  and  Torreya. 
If,  however,  the  roots  in  the  two  genera  be  examined,  a  varying 
amount  of  storage  parenchyma  is  discovered  which  is  ordinarily 
better  developed  in  proximity  to  the  region  of  the  primary  wood 
(Fig.  253).  In  species  common  to  Europe  and  America,  namely, 

Taxus  baccata  and  its  vari- 
eties, very  little  paren- 
chyma is  found  even  in  the 
root;  but  in  oriental  species 
of  the  genus  storage  ele- 
ments are  somewhat  abun- 
dant in  this  organ.  The 
young  stem,  and  particu- 
larly the  root,  of  the  genus 
Torreya  show  clearly 
developed  longitudinal 
parenchyma  in  the  wood. 
In  Cephalotaxus,  which  by 
reason  of  the  presence  of  a 
well-developed  female  cone 


FIG.  252. — Transverse  section  of  the  wood 
of  the  stem  in  Taxus  cuspidata. 


at  an  early  stage  must  be 
regarded  as  a  primitive  genus  of  the  subtribe,  parenchyma  is 
markedly  abundant  in  the  organization  of  the  wood.  Injuries, 
also,  frequently  result  in  the  recall  of  parenchyma  in  those  taxineous 
woods  which  are  normally  without  it.  The  generally  uniseriate 
rays  of  the  subtribe  do  not  show  the  presence  of  traumatic  ray- 
tracheids  as  a  result  of  injury,  and  in  this  respect  they  present  a 
marked  resemblance  to  the  woods  of  the  Podocarpineae.  Resin 
canals  are  conspicuously  absent  in  the  ligneous  structures  of  the 
Taxineae,  and  not  the  slightest  evidence  of  their  former  occurrence 
can  be  produced  by  experimental  data.  The  genus  Taxus  is 
entirely  without  resin  canals,  even  in  its  leaves,  thus  providing 
the  only  example  of  a  conifer  completely  lacking  these  structures. 
An  interesting  parallel  is  presented  by  the  hemlock  (Tsuga)  among 


CONIFERALES 


347 


the  Abietineae.  Here  resin  canals  are  normally  absent,  not  only 
in  the  structure  of  the  wood,  but  also  in  the  other  tissues  of  the 
stem.  In  the  root  of  the  genus  they  occur  in  the  primary  wood; 
and  in  the  cone  and  leaves,  in  the  tissues  of  the  cortex.  This 
distribution  of  resin  canals  is  of  course  entirely  in  accord  with 
the  general  principles  of  evolutionary  anatomy.  Tsuga,  however, 
differs  markedly  from  Taxus  in  the  degree  of  obliteration  of  resin 
canals,  since  these  structures  here  persist  in  conservative  regions, 
while  in  the  latter  genus 
they  have  entirely  disap- 
peared. In  Cephalotaxus 
resin  canals  are  well  devel- 
oped except  in  the  wood, 
but  are  apparently  not 
susceptible  of  reversionary 
recall  in  ligneous  struc- 
tures. 

It  will  be  obvious  from 
the  description  of  the  re- 
productive and  vegetative 
organization  supplied  in 
the  two  preceding  para- 
graphs that  so  simple  a 


FIG.  253. — Transverse  section  of  the  wood 
of  the  root  in  Taxus  cuspidata  (after  Bliss). 


type  as  the  genus  Taxus 
cannot,  according  to  the  well-established  principles  of  anatomy,  be 
regarded  as  a  primitive  form.  Its  acceptance  in  this  capacity  by 
the  earlier  and  philosophical  taxonomy  is  shown  on  inductive  evi- 
dence to  be  entirely  unjustified.  It  appears  on  the  basis  of  the 
facts  at  present  available  that  the  Taxineae  are  a  reduction  series 
in  which  the  oriental  Cephalotaxus  among  existing  genera  occupies 
the  lowest  position,  while  Taxus  represents  the  summit.  It  is 
further  obvious  that  the  absence  of  wood  parenchyma  is  not  a 
primitive  feature  of  organization  of  the  group.  If  it  be  admitted 
that  longitudinal  parenchymatous  elements  primitively  character- 
ized the  organization  of  the  wood,  a  derivation  from  the  same 
stock  as  the  Podocarpineae  is  plainly  indicated.  This  conclusion 
is  justified  by  the  consideration  of  the  young  ovuliferous  cone  in 


348  THE  ANATOMY  OF  WOODY  PLANTS 

the  genus  Cephalotaxus.  The  anatomical  evidence  points  very 
clearly  to  the  Taxineae  as  a  reduction  series,  taking  its  origin  from 
the  same  general  group  or  plexus  as  the  Podocarpineae.  The 
somewhat  meager  fossil  data  at  our  disposal  do  not  warrant 
us  in  assuming  a  very  ancient  occurrence  for  the  subtribe  Taxineae, 
since  woods  of  this  type  are  not  found  earlier  than  the  Tertiary 
or  Cenozoic.  It  is  true  that  cones  and  leafy  twigs  from  various 
levels  of  the  Mesozoic  have  from  time  to  time  been  referred  to 
taxineous  affinities,  but  there  is  no  anatomical  evidence  that  they 
belonged  here.  So  far  as  any  data  derived  from  anatomical 
structure  are  concerned,  we  are  not  justified  in  attributing  to 
the  Taxineae  a  great  geological  age.  The  external  habit  as  a 
diagnostic  criterion  in  the  Coniferales  has  been  shown  in  recent 
years  to  be  almost  as  misleading  as  it  has  proved  to  be  in  the 
Cycadofilicales  and  other  Paleozoic  groups. 

Having  considered  the  anatomical  organization  of  the  various 
subtribes  of  the  Coniferales  in  such  detail  as  comports  with  an 
elementary  work  like  the  present,  before  summing  up  the  situation 
from  the  evolutionary  standpoint  we  may  well  make  some  refer- 
ence to  the  anatomical  characteristics  of  coniferous  woods  which 
are  utilized  by  paleobotanical  investigators.  The  genera  of  fossil 
woods  logically  increase  in  number  as  our  knowledge  of  the  ligneous 
organization  of  extinct  plants  becomes  fuller  and  more  complete. 
For  the  present  purpose  only  a  few  of  the  more  important  genera 
of  fossil  woods  need  be  considered.  By  reason  of  its  relative 
resistance  to  decay,  wood  naturally  often  becomes  isolated  from 
its  accompanying  tissues  and  frequently  is  the  sole  surviving  evi- 
dence of  the  former  existence  of  gymnospermous  groups.  The 
situation  thus  presented  is  a  difficult  one,  and  the  earlier  charac- 
terizations of  fossil  wood  were  naturally  largely  empirical  and 
made  without  reference  to  the  facts  or  principles  of  comparative 
anatomy. 

In  deference  to  the  prevailing  view  that  the  araucarian  conifers 
are  the  most  ancient  and  form  the  connecting  link  between  the 
Coniferales  and  the  Cordaitales,  we  may  consider  them  first. 
It  has  been  indicated  in  a  previous  paragraph  that  the  mature 
secondary  wood  of  the  two  living  genera  of  the  Araucariineae  is 


CONIFERALES  349 

characterized  by  alternating  crowded  pitting  and  the  absence  of 
bars  of  Sanio  as  well  as  of  parenchymatous  storage  elements.  To 
this  type  of  wood  occurring  as  a  fossil  the  name  Araucarioxylon 
is  given.  A  second  genus  of  fossil  woods  is  diagnosed  by  the 
presence  of  spirals  on  the  inner  walls  of  the  tracheids  and  by  the 
absence  of  wood  parenchyma,  under  the  generic  designation 
Taxoxylon.  Those  woods  characterized  by  the  possession  of 
ligneous  resin  canals  in  the  horizontal  and  vertical  planes  are 
included  under  the  generic  appellation  Pityoxylon.  In  coniferous 
wood  without  either  resin  canals  or  conspicuous  storage  paren- 
chyma, the  name  Cedroxylon  is  adopted.  Woods  which,  on  the 
contrary,  are  provided  with  abundant  storage  parenchyma  are 
designated  Cupressinoxylon.  In  all  genera  of  fossil  woods  except 
the  first  the  pitting  is  characteristically  opposite.  In  the 
Araucarioxylon  type  the  pitting  is  frequently  crowded  and 
alternation  and  bars  of  Sanio,  universally  present  in  other  existing 
coniferous  woods,  are  conspicuous  by  their  absence. 

Clearly  the  use  of  even  the  small  number  of  genera  of  fossil 
woods  indicated  in  the  preceding  paragraph  in  connection  with 
evolutionary  inferences  must,  in  the  light  of  the  conditions  de- 
scribed for  the  various  living  conifers  in  the  earlier  part  of  the 
present  chapter,  be  a  matter  of  great  difficulty.  This  arises  from 
the  fact  that  the  organization  of  the  mature  wood  in  a  given 
conifer  is  by  no  means  necessarily  an  indication  of  its  true  system- 
atic position.  The  interpretation  of  the  significance  of  wood 
structure  in  fossil  and  existing  conifers  can  be  successfully  attacked 
only  with  a  knowledge  of  the  general  principles  of  comparative 
anatomy.  A  failure  to  realize  this  situation  has  led  to  very  many 
erroneous  interpretations,  both  anatomical  and  paleobotanical. 
For  example,  it  is  quite  clear  from  the  data  supplied  at  an  earlier 
stage  that  the  primitive  condition  of  both  the  Araucarioxylon 
and  Taxoxylon  type  was  a  Cupressinoxylon,  since  abundant  wood 
parenchyma  diffused  throughout  the  annual  ring  was  formerly  a 
feature  of  organization  of  these  ligneous  types,  as  is  shown  by 
a  consideration  of  conservative  organs,  experimental  results,  and 
fossil  evidence.  Further,  in  some  instances  the  Cupressinoxylon 
type  has  been  clearly  derived  from  the  complicated  structure 


350  THE  ANATOMY  OF  WOODY  PLANTS 

found  in  Pityoxylon.  This  is,  for  example,  true  in  the  case  of 
the  genus  Sequoia,  which,  on  the  basis  of  general  anatomical 
principles,  obviously  formerly  possessed  both  the  ligneous  resin 
canals  and  the  marginal  ray-tracheids  of  the  older  Abietineae. 
Examples  might  be  indefinitely  multiplied  to  show  that  the  use 
of  the  mature  structure  of  the  wood  in  the  Coniferales,  without 
recourse  to  comparative  anatomical  and  experimental  data,  is 
almost  certain  to  lead  in  a  given  case  to  fallacious  conclusions. 

It  is  now  possible  to  sum  up  the  situation  from  the  standpoint 
of  evolutionary  anatomy  for  the  phylogenetic  sequence.  It  will 
be  convenient  to  indicate  preliminarily  that  the  most  generally 
accepted  hypothesis  of  the  morphological  nature  of  the  ovuliferous 
cone  in  the  Coniferales  furnishes  a  prima  facie  argument  in  favor 
of  the  primitive  position  of  the  Abietineae.  In  this  subtribe  the 
female  cone  consists  of  pairs  of  scales,  the  upper  of  which  is  ovulif- 
erous and  the  lower  sterile.  The  ovuliferous  scale  is  with  a  strong 
degree  of  probability  to  be  regarded  as  a  persistent  single  sporophyll 
bearing  two  seeds  or  megasporangia  on  its  morphologically  lower 
but  physically  upper  surface.  The  megasporophyll  is  in  relation 
to  the  abortive  axis  of  the  reproductive  short-shoot.  Aside  from 
the  morphological  interpretation  of  the  structures  concerned, 
however,  the  fact  remains  that  the  units  of  structure  in  the  ovulif- 
erous cone  of  the  Abietineae  are  double  and  separate  in  their 
nature.  In  the  female  cones  of  the  remaining  coniferous  sub- 
tribes  there  is  clear  evidence  from  comparative  anatomy  of  the 
presence  of  fused  pairs  of  scales  in  the  ovuliferous  structures. 
In  accordance  with  general  principles  of  morphology  and  without 
necessarily  accepting  the  hypothesis  of  the  short-shoot  nature 
of  the  vertically  paired  scales  of  the  abietineous  cones,  it  is  probable 
that  the  Abietineae  are  an  older  group  than  are  the  remaining 
coniferous  sub  tribes. 

An  additional  argument  for  the  antiquity  of  the  Abietineae 
is  furnished  by  their  obviously  close  relationship  to  the  Ginkgoales, 
which  are  admitted  on  every  hand  to  be  a  primitive  group  of 
gymnosperms.  The  affinity  with  the  Ginkgoales  shows  itself 
in  the  common  possession  of  vegetative  short-shoots  by  both  the 
sole  surviving  Ginkgo  and  by  the  ancient  but  still  prolific  genus 


CONIFERALES  351 

Pinus.  Further,  both  megasporophylls  and  microsporophylls 
in  the  two  cases  produce  paired  sporangia.  The  microsporangia 
of  Ginkgo  present  a  common  difference  with  those  of  the  Abie- 
tineae  from  lower  forms  in  owing  their  dehiscence  to  a  mechanical 
layer  derived,  not  from  the  epidermal,  but  from  the  fibrovascular, 
tissues.  An  additional  feature  of  affinity  is  supplied  by  the 
winged  character  and  the  internal  organization  of  the  microspores 
in  the  two  groups,  for  they  are  practically  identical.  The  structure 
of  the  tracheids  of  the  wood  in  the  Abietineae  and  Ginkgoales  is 
significantly  similar  in  the  presence  of  opposite  pitting  and  bars 
of  Sanio,  a  common  feature  which  distinguishes  them  from  the 
Cordaitales  and  other  older  gymnospermous  groups.  Further, 
although  the  pitting  and  other  details  of  organization  of  the 
tracheids  in  the  two  groups  is  of  the  modern  gymnospermous 
type,  the  structure  of  the  xylem  in  primitive  regions  clearly  shows 
a  filiation  with  the  conditions  characteristically  presented  by  the 
Paleozoic  gymnosperms. 

If  a  strong  argument  for  the  primitive  position  of  the  Abie- 
tineae is  supplied  by  a  comparison  with  the  structural  features  of 
the  Ginkgoales,  an  even  more  cogent  one  is  furnished  by  their 
resemblance  in  important  anatomical  characteristics  to  the  Cor- 
daitales. It  is  the  leaf  of  the  extinct  genus  Prepinus  which  mani- 
fests, as  has  been  indicated  in  earlier  pages,  the  most  categorical 
and  distinct  similarity  to  the  foliar  organization  of  certain  Cor- 
daitales. Not  only  is  Prepinus  the  only  known  representative 
of  the  Coniferales  to  show  distinct  and  unmistakable  centripetal 
wood  as  distinguished  from  transfusion  tissue,  but  it  manifests 
its  affinity  to  cordaitean  forms  by  the  presence  of  a  double  trans- 
fusion sheath  in  relation  to  the  centripetal  wood  of  the  foliar  bundle. 
It  is  further  impossible  to  deny  for  Prepinus  a  close  degree  of 
relationship  with  Cretaceous  species  of  Pinus  which,  like  Prepinus, 
possess  a  well-marked  double  transfusion  sheath  and  differ  from 
the  more  primitive  genus  only  in  the  absence  of  true  centripetal 
wood.  The  agreement  in  foliar  organization  between  Prepinus 
and  cretaceous  species  of  Pinus  on  the  one  hand  and  the  Cordaitales 
on  the  other  cannot  be  overlooked  in  any  discussion  of  the  evo- 
lution and  affinities  of  coniferous  subtribes.  Although  the  wood 


352  THE  ANATOMY  OF  WOODY  PLANTS 

of  Pinus  and  Prepinus  resembles  that  of  Ginkgo  in  possessing 
opposite  pitting  and  bars  of  Sanio,  this  organization  contrasting 
with  the  tracheary  structure  of  older  types  like  the  Cordaitales 
and  the  Cycadales  in  reality  presents  no  difficulty.  For  hi  prim- 
itive regions  and  organs  the  alternating  pitting  without  bars  of 
Sanio  characteristic  of  the  Cordaitales  is  present  in  the  Abietineae 
(particularly  in  Pinus  and  Prepinus),  and  passes  by  gradual 
transitions  into  the  opposite  pitting  with  bars  of  Sanio  exemplified 
in  the  structure  of  the  mature  wood  both  in  Ginkgo  and  in  the 
Abietineae.  The  reproductive  structures  of  the  Cordaitales  are 
too  little  known  to  us  to  supply  appropriate  points  of  comparison. 
It  may  be  stated  in  summary  that  the  Abietineae  present  as  good 
anatomical  evidence  for  derivation  from  the  Cordaitales  as  do 
the  Ginkgoales;  and,  moreover,  they  have  an  additional  claim  to 
such  affinity  by  their  clear  relationship  with  the  Ginkgoales. 

But  it  is  not  only  by  reason  of  resemblances  to  the  Ginkgoales 
and  the  Cordaitales,  neither  few  nor  unimportant,  that  the  Abie- 
tineae show  themselves  to  be  a  primitive  subtribe  of  the  Conif erales. 
When  the  anatomical  organization  of  the  remaining  coniferous 
subtribes  is  investigated  in  the  light  of  the  general  principles  so 
often  emphasized  in  the  present  volume,  it  supplies  in  many  cases 
distinct  evidence  that  the  Abietineae  are  the  primitive  stock  from 
which  all  have  taken  their  origin.  This  situation  may  be  illus- 
trated first  in  the  case  of  the  Araucariineae,  which  are  somewhat 
generally  regarded  as  the  primitive  conifers  and  those  most  nearly 
connected  with  the  Cordaitales.  The  strongest  evidence  for  this 
claim  is  supplied  by  the  organization  of  the  tracheids,  presenting 
the  same  alternating  pitting  and  the  absence  of  bars  of  Sanio  as 
are  manifested  in  the  cordaitean  and  cycadalian  gymnosperms. 
If  alternating  pitting  alone  were  a  sufficient  criterion  of  relation- 
ship, many  angiosperms  by  the  possession  of  this  feature  could 
establish  a  claim  to  relationship  with  that  ancient  group  of  gymno- 
sperms. The  evidence  here  as  in  other  cases  should  be  read  in 
the  light  of  the  general  principles  of  comparative  anatomy.  The 
longitudinal  radial  section  of  the  cone  axis,  and  to  a  lesser  extent 
of  the  leaf  trace,  of  either  Araucaria  or  Agathis  at  once  reveals 
the  true  situation.  In  these  cases  one  always  finds  bars  of  Sanio 


CONIFERALES  353 

in  the  region  of  the  secondary  wood  near  the  primary  xylem,  and, 
where  the  pitting  is  abundant,  frequent  opposition  of  the  pores. 
In  other  words,  a  situation  presents  itself  which  is  the  exact  con- 
verse of  that  found,  for  example,  in  the  Ginkgoales,  in  which  the 
primitive  region  of  the  secondary  wood  shows  alternating  pitting 
and  no  bars  of  Sanio.  In  the  later-formed  wood  the  pits  begin 
to  appear  in  opposition  and  are  separated  by  bars  of  Sanio.  If 
we  are  justified  in  regarding  the  type  of  tracheid  found  in  Ginkgo 
as  originating  from  that  typical  of  Cordaitales  and  other  ancient 
gymnosperms,  we  are  similarly  warranted  on  the  same  evidence 
in  viewing  the  alternation  of  pitting  and  the  absence  of  bars  of 
Sanio  in  the  adult  wood  of  the  existing  araucarian  conifers  as 
derived  from  a  state  in  which  both  opposition  of  pitting  and  bars 
of  Sanio  were  present.  The  evidence  supplied  by  fossil  forms, 
moreover,  justifies  the  derivation  of  the  Araucariineae  from  abie- 
tineous  forbears,  since  many  distinctly  araucarian  woods  have 
been  discovered  in  recent  years  in  the  Mesozoic  which  clearly 
present  either  normal  or  traumatic  features  uniting  them  with 
the  Abietineae.  In  some  instances,  for  example,  thick-walled, 
heavily  pitted  ray  cells,  similar  to  those  commonly  characteristic 
of  radial  parenchyma  in  living  and  fossil  Abietineae,  are  found  in 
Mesozoic  araucarian  woods.  This  feature  is  significantly  paral- 
leled in  the  rays  of  the  cones  of  living  araucarians.  Traumatic 
resin  canals  are  also  found  commonly  in  woods  which  are  transi- 
tional from  the  abietineous  to  the  araucarian  type.  This  is  notably 
the  case  in  Brachyoxylon,  a  very  common  Mesozoic  type  of  wood, 
and  also  in  the  much  rarer  Araucariopitys.  Normal  resin  canals 
are,  moreover,  found  in  the  wood  of  the  ovuliferous  cone  of  the 
living  Agathis  Bidwillii,  from  Java.  It  has  been  stated  in  earlier 
pages  that  there  is  distinct  evidence  from  the  organization  of 
actual  fossils  and  from  the  structure  of  conservative  regions  in 
the  existing  Araucariineae  that  abundant  wood  parenchyma  was 
an  original  feature  of  organization  of  the  araucarian  as  contrasted 
with  the  abietineous  Coniferales  and  the  Ginkgoales,  in  which 
tangential  storage  elements  are  conspicuously  absent. 

Not  only,  however,  have  abietineous  forms  obviously  given 
rise  to  the  araucarian  stock  on  the  basis  of  the  general  principles  of 


354  THE  ANATOMY  OF  WOODY  PLANTS 

comparative  anatomy  and  the  organization  of  fossil  forms,  but  the 
same  derivation  is  likewise  indicated  by  two  other  series — the 
Taxodineae-Cupressineae  on  the  one  hand  and  the  Podocarpineae- 
Taxineae  on  the  other.  The  first  series,  on  the  basis  of  com- 
parative anatomical  evidence,  formerly  possessed  the  ligneous 
resin  canals  and  the  marginal  ray-tracheids  of  the  later  Mesozoic 
Abietineae.  The  supposed  species  of  Sequoia  of  the  Mesozoic 
are  not  representative  of  the  living  genus,  but  have  the  organiza- 
tion of  araucarian  conifers,  as  has  been  shown  by  recent  anatomical 
investigation  of  the  forms  from  the  American  Cretaceous.  They 
accordingly  have  no  bearing  on  the  question  of  the  origin  of  the 
Taxodineae-Cupressineae.  In  the  case  of  the  series  which  has 
been  described  above  as  the  Podocarpineae-Taxineae,  the  evidence 
from  fossil  forms  is  practically  non-existent  in  the  present  state 
of  our  ignorance  in  regard  to  the  organization  of  the  Mesozoic 
conifers  of  the  Southern  Hemisphere.  The  situation  must  there- 
fore be  judged  on  the  basis  of  the  living  forms.  The  reproductive 
structures  of  the  Podocarpineae,  particularly  those  of  the  genus 
Podocarpus,  are  strikingly  abietineous  and  sufficiently  clearly 
indicate  the  affinities  of  the  subtribe.  The  primitive  Taxineae, 
obviously  in  accordance  with  the  established  principles  of  com- 
parative anatomy,  formerly  possessed  the  abundant  wood  paren- 
chyma of  the  Podocarpineae,  and  their  systematic  position  is 
therefore  elucidated.  If  the  statements  in  the  present  paragraph 
are  well  founded,  evidently  both  the  Taxodineae-Cupressineae 
and  the  Podocarpineae-Taxineae  are  of  abietineous  origin. 

The  time  has  now  come  to  summarize  the  phylogenetic  affini- 
ties of  the  Coniferales,  both  as  regards  the  general  relationship 
of  the  tribe  and  as  regards  the  affinities  of  its  particular  subtribes 
with  one  another.  It  seems  clear  that  the  Abietineae  have  on 
all  counts  the  strongest  claim  to  be  considered  as  primitive  repre- 
sentatives of  the  Coniferales.  These  may  be  summarized  as 
follows:  filiation  with  the  Cordaitales  and  co-ordination  with  the 
Ginkgoales;  precedence  to  the  Araucariineae,  to  the  Taxodineae- 
Cupressineae,  and  to  the  Podocarpineae-Taxineae.  The  evidence 
for  the  ancestral  character  of  the  abietineous  conifers  may  in 
the  future  be  fuller,  but  scarcely  any  stronger,  than  it  is  at  the 


CONIFERALES 


355 


present  time.  The  accompanying  genealogical  tree  (Fig.  254) 
will  make  clear  the  views  as  to  sequence  and  affinities  developed 
in  the  previous  paragraphs  of  the  present  chapter.  The  Cordaitales 
of  the  Paleozoic  serve  as  the  starting-point,  and  from  them  were 
derived  two  cognate  stocks— the  Coniferales  and  the  Ginkgoales. 
The  latter  have  suffered  much  extinction  and  end  in  the  present 
epoch  in  the  sole  surviving  genus,  Ginkgo.  The  Abietineae  in 
earlier  Mesozoic  time 
gave  rise  to  the 
Araucariineae,  which 
flourished  greatly  in 
the  later  epochs  of 
that  period,  to  become 
almost  extinct  in  the 
glaciation  which 
ushered  in  the  Ter- 
tiary. The  arauca- 
rian  conifers  dis- 
tinctly took  their 
origin  from  the 
abietineous  stock 
after  it  had  developed 
ligneous  resin  canals, 
but  before  marginal 
ray-tracheids  had 
made  their  appearance.  The  Podocarpineae-Taxineae  may  have 
originated  at  a  still  earlier  epoch  and  before  the  abietineous  stock 
had  developed  the  resin  canals  or  the  marginal  ray-tracheids  which 
distinguish  it  from  the  Cretaceous  onward.  Unlike  the  Arau- 
cariineae the  Podocarps  and  their  allies,  the  yews,  did  not  lose  the 
opposite  pitting  and  bars  of  Sanio  common  to  the  parent  stock  of 
both  Abietineae  and  Ginkgoales.  The  Taxodineae  are  a  still  later 
offshoot  of  the  strong  abietineous  line  and  come  into  existence  sub- 
sequent to  the  appearance  of  marginal  ray-tracheids  and  ligneous 
resin  canals.  Certain  conifers  of  the  earlier  and  later  Mesozoic,  such 
as  Voltzia,  Brachyphyllum,  Geinitzia  (sometimes  erroneously  desig- 
nated Sequoia),  etc.,  which  have  been  referred  to  taxodineous 


?ora!(7/fa/es 
FIG.  254. — Genealogical  tree  of  the  Coniferales, 
showing  their  proximity  to  the  Ginkgoales. 


356  THE  ANATOMY  OF  WOODY  PLANTS 

affinities,  in  reality  have  nothing  to  do  with  that  group,  but  are 
araucarian  or  pre-araucarian  in  their  relationships.  There  is  at  the 
present  time  no  trustworthy  evidence  that  the  Taxodineae  were  in 
existence  before  Tertiary  times,  although  it  is  quite  possible  in  view 
of  the  general  situation  that  they  made  their  appearance  in  the 
later  Cretaceous.  The  Cupressineae  must  be  regarded  as  a  con- 
tinuation of  the  taxodineous  line  and  as  having  a  similar  relation 
to  the  abietineous  ancestral  forms. 

In  conclusion,  it  may  be  remarked  that,  whatever  may  be 
the  differences  of  opinion  in  regard  to  the  reading  of  the  evolution- 
ary document  supplied  by  the  Coniferales  as  they  now  present 
themselves  to  our  gaze  or  are  preserved  for  us  as  fossils  from 
earlier  geological  epochs,  there  can  be  no  doubt  that  they  con- 
stitute the  most  important  of  all  documents  for  the  develop- 
ment of  general  evolutionary  principles  as  the  result  of  inductive 
reasoning.  The  treatment  of  the  group  in  the  present  and 
preceding  chapters  is  intended  to  clear  the  way  for  their 
further  study  by  the  development  of  general  situations  in  relation 
to  particular  anatomical  and  paleobotanical  facts.  A  continued 
investigation  of  the  group,  for  which  our  American  Mesozoic 
deposits  have  already  yielded  so  much  material  of  crucial  impor- 
tance, is  likely  to  result  in  the  firm  establishment  of  extremely 
valuable  general  principles  for  that  type  of  biological  research 
which  bases  its  conclusions  on  inductive  reasoning  rather  than 
on  any  purely  philosophical  attitude,  either  mechanistic  or  vital- 
istic. 


CHAPTER  XXV 

THE  METAGYMNOSPERMAE:   GNETALES 

The  aggregation  of  forms  included  in  the  present  chapter  is 
very  small,  but  is  of  great  importance  from  the  phylogenetic 
standpoint.  The  Gnetales  are  represented  in  the  existing  flora 
of  our  earth  by  three  genera.  Of  these  the  genus  Ephedra  occurs 
throughout  the  Northern  Hemisphere,  being  somewhat  abundant 
in  the  American  and  Asiatic  continents  and  rare  in  Europe.  Gnetum 
is  a  characteristically  vinelike  form  occurring  in  the  eastern  and 
western  tropics.  The  third  genus,  Welwitschia,  is  monotypic 
and  is  confined  to  the  southwestern  region  of  the  African  continent. 
Of  the  three  generic  types  enumerated,  Ephedra  must  be  consid- 
ered on  account  of  both  its  reproductive  and  its  vegetative  features 
as  on  the  whole  the  most  primitive,  although  naturally  its  desert 
habitat  has  exerted  more  or  less  influence  on  its  internal  organiza- 
tion. Gnetum  in  general  represents  the  highest  stage  of  develop- 
ment attained  in  the  group,  and  this  statement  is  particularly 
true  of  the  features  presented  by  the  organization  of  its  gameto- 
phytes.  Anatomically,  however,  Gnetum  seems  to  occupy  a 
somewhat  less  specialized  position  than  does  the  extremely  xero- 
phytic  Welwitschia.  The  South  African  genus  just  named  in  its 
gametophytic  organization  is  in  a  general  way  intermediate 
between  Ephedra  and  Gnetum.  The  consideration  of  the  repro- 
ductive structures  proper,  using  that  designation  to  cover  both 
the  floral  organization  and  the  gametophytes,  need  not  be  cov- 
ered more  than  incidentally  in  the  present  volume,  since  the 
morphology  of  the  parts  related  to  sex  has  recently  been  fully 
discussed  in  Coulter  and  Chamberlain's  Morphology  of  Gymno- 
sperms. 

A  transverse  section  of  a  stem  of  Ephedra  (Fig.  255)  reveals  an 
anatomical  situation  not  found  in  any  of  the  Coniferales.  Here  the 
woody  cylinder  is  characterized  by  rays  which  become  very  broad  in 
the  outer  region  of  the  wood  but  are  quite  narrow  in  the  vicinity 

357 


358 


THE  ANATOMY  OF  WOODY  PLANTS 


of  the  pith.  Not  only  is  the  structure  of  the  wood  in  the  genus 
under  discussion  contrasted  in  the  nature  of  its  radial  parenchyma 
with  that  of  the  Coniferales,  but  it  also  presents  a  striking  resem- 
blance to  the  dicotyledons  in  the  possession  of  true  vessels,  albeit 
of  a  primitive  type.  The  correlation  of  large  rays  and  true  vessels 
in  the  organization  of  the  wood  in  both  Gnetales  and  dicotyledons 
is  a  feature  which  is  clearly  not  without  evolutionary  significance, 
as  will  be  shown  in  subsequent  pages.  The  pith  is  of  moderate 

size  and  the  phloem 
and  cortex  constitute 
a  rather  thin  layer 
on  the  surface  of  the 
strongly  convoluted 
woody  cylinder.  The 
depressions  on  the 
face  of  the  wood 
plainly  correspond  to 
the  position  of  the 
large  rays,  as  is  the 
case  with  identical 
conditions  in  certain 
dicotyledons. 

In    Fig.    256    is 

FIG.    255.-Transverse   section  of   the   stem  of      shown     a     transverse 
Ephedra  gerardiana.  View  of  the  Structure 

of  the  wood  some- 
what highly  magnified.  The  features  presented  by  the  vessels 
and  rays  stand  out  very  clearly.  It  is  obvious  that  annual  rings 
are  present,  although  not  conspicuously  developed.  The  paren- 
chyma is  of  diffuse  distribution  as  in  the  higher  conifers.  The 
large  rays,  however,  illustrate  interesting  and  phylogenetically 
important  features.  Even  in  the  transverse  view  they  are  clearly 
composite  structures  and  do  not  consist  homogeneously  of  paren- 
chyma, as  in  the  corresponding  radial  bands  in  our  northern 
oaks  and  in  the  genus  Gnetum.  Vessels  are  seen  intimately 
incorporated  in  the  organization  of  the  ray.  The  vascular  are 
not  the  only  elements  of  the  longitudinal  structure  of  the  wood 


GNETALES 


359 


incorporated  in  the  large  ray,  for  fibers  are  also  present,  although 
they  cannot  be  so  weU  recognized  in  the  transverse  section.  The 
constitution  and 
origin  of  the  large 
rays  in  Ephedra  are 
of  great  signifi- 
cance, not  only  for 
the  Gnetales  them- 
selves, but  also  in 
connection  with  the 
problem  of  the  evo- 
lution of  the  higher 
forms  known  as 
dicotyledonous  an- 
giosperms.  It  will 
accordingly  be  con- 
sidered somewhat 
in  detail. 

Fig  •  257  illus-  pIG-  256.— Transverse  section  of  the  wood  of  Ephedra 

species. 

tion  of  the  large  ray 
in  an  early  stage  of 
development  when 
it  is  still  rather  nar- 
row and  close  to  the 
medullary  region  of 
the  stem.  An  in- 
spection of  the  illus- 
tration  makes  it 
clear  that  the  ray  is 
by  no  means  a 
homogeneous  struc- 
ture  composed 
entirely  of  storage 
parenchyma.  Fi- 

FXG.  2S7.-Longitudinal  section  of  a  large  ray  of     br°US      elements 
Ephedra  in  proximity  to  the  pith.  necessarily  enter 


trates  the  organiza- 


36° 


THE  ANATOMY  OF  WOODY  PLANTS 


largely  into  its  composition,  and  these  are  often  in  the  condition 
of  septation.  The  ray,  in  fact,  is  a  composite  structure,  organized 
only  partially  from  true  radial  parenchyma  and  also  consisting 
largely  of  transformed  longitudinal  fibrous  elements  of  the  wood. 
These  first  become  septate,  and,  particularly  in  the  more  external 
regions  of  the  wood,  their  divisions  become  progressively  more  and 
more  like  the  ordinary  storage  elements  of  the  ray.  Fig.  258  por- 
trays the  longitu- 
dinal organization 
of  the  wood  in  the 
outer  region  of  a 
rather  thick  stem  of 
Ephedra  calif ornica. 
The  large  rays  are 
here  conspicuous 
and  numerous,  but 
not  of  equal  size. 
In  general,  those  of 
greater  dimensions 
have  originated  in 
the  region  of  the 
medulla,  while 
those  less  conspicu- 
ous by  their  size 
have  come  into  ex- 
istence  more  re- 
cently. The  small  degree  of  magnification  employed  in  the  figure 
does  not  make  it  possible  to  discern  clearly  the  composite  character 
of  the  radial  parenchyma.  The  next  illustration,  Fig.  129,  which 
reproduces  one  of  the  smaller  radial  masses  under  a  higher  magnifica- 
tion, makes  the  organization  of  these  structures  apparent.  Obvi- 
ously not  only  ordinary  radial  parenchyma  is  concerned  in  the 
constitution  of  the  rays,  but  also  numerous  fibers  and  even  vessels. 
It  may  here  be  stated,  although  that  situation  is  not  clearly  shown 
in  the  illustration,  that  fibers  are  seen  in  such  rays  in  all  conditions 
of  transformation  into  elements  resembling  the  ordinary  radial 
parenchyma.  In  the  genus  Ephedra  we  have  the  wedding,  as 


FIG.  258. — Tangential  view  of  a  large  ray  of  Ephedra 
in  its  external  region. 


GNETALES 


it  were,  of  radial  and  longitudinal  storage  devices  to  constitute  a 
new  and,  from  the  evolutionary  standpoint,  an  extremely  important 
type  of  radial  organization.  A  fact  not  without  significance  in 
the  present  connection  is  the  correlation  of  vessels  with  the  more 
abundant  storage  devices  present  in  the  wood  of  the  Gnetales. 
The  large  ray  in  the  Gnetales  is  evidently  a  composite  derived 
from  the  fusion  in  certain  radii  of  the  wood  of  the  original  radial 
storage  paren- 
chyma with  paren- 
chymatous  ele- 
ments derived  from 
the  copious  trans- 
formation of  the 
longitudinal  tra- 
cheary  elements  of 
the  wood  by  septa- 
tion.  As  has  been 
indicated  in  earlier 
pages,  wood  paren- 
chyma made  its 
appearance  in  the 
earlier  and  lower 
Coniferales  first  in 
relation  to  the  end 
of  the  annual  ring 

and  in  Mesozoic  times,  when  well-marked  annual  periods  of  vege- 
tative inactivity  had  become  established.  Subsequently  the  wood 
parenchyma,  in  its  inception  clearly  derived  from  the  septation  of 
tracheids,  became  diffused  throughout  the  annual  ring.  When  it 
had  become  abundant  and  well  established  in  this  condition,  a 
situation  favorable  to  the  appearance  of  large  rays  was  attained. 
The  final  impetus  to  the  appearance  of  these  structures  was 
supplied  by  the  origin  of  vessels  which,  by  providing  a  greater 
supply  of  food  substances  and  water  from  the  soil,  rendered 
possible  larger  and  more  efficient  leaves.  These  in  turn  pro- 
duced larger  quantities  of  assimilates  which  by  the  appearance 
of  the  large  rays  (co-ordinate  in  their  origin  with  vessels  and 


FIG.  259. — Another  view  of  the  same 


362 


THE  ANATOMY  OF  WOODY  PLANTS 


diffuse  wood  parenchyma)  found  storage  in  the  woody  tissues 
of  the  axis. 

It  will  be  obvious  from  the  facts  brought  forward  in  the  pre- 
ceding paragraph  that  diffuse  and  abundant  wood  parenchyma, 
vessels,  and  large  rays  are  features  which  are  -intimately  corre- 
lated in  the  organization  of  the  woods  of  the  Gnetales  and  the 
dicotyledons.  That  the  situation  portrayed  is  correct  from  the 

evolutionary  stand- 
point is  rendered 
clear  by  a  condi- 
tion presented  not 
infrequently  in  the 
wood  of  our  living 
species  of  Pinus. 
Fig.  260  illustrates 
an  interesting  ab- 
normal feature 
which  is  often  found 
in  the  wood  of  the 
white  pine.  The 
annual  rings  are 
strongly  depressed 
locally,  and  in  these 
regions  the  rays  of 
the  wood  are 

unusually  abundant,  or,  in  other  words,  are  clustered  or  aggregated. 
Since  in  the  genus  Pinus  wood  parenchyma  is  absent — a  feature, 
as  has  been  indicated  in  earlier  pages,  definitely  correlated  with 
its  primitive  phylogenetic  position — the  clustering  of  the  rays, 
particularly  in  the  absence  of  vessels,  is  of  no  evolutionary  signifi- 
cance. This,  of  course,  is  primarily  the  result  of  the  absence  of 
the  later  acquired  capacity  of  producing  the  longitudinal  paren- 
chyma so  necessary  for  the  fusion  of  the  aggregation  of  rays  into 
large  and  homogeneous  storage  units.  It  may  further  be  remarked 
in  a  general  way  that  there  is  no  evidence  to  show  that  the  large 
masses  of  storage  parenchyma  which  are  so  striking  a  feature 
of  the  organization  of  the  woody  cylinder  of  both  the  Gnetales 


FIG.  260. — Aggregate  ray  from  Pinus  Strobus 


GNETALES 


363 


and  the  dicotyledons  were  in  the  first  instance  related  to  the 
appendages,  whether  branch,  leaf,  or  root.  While,  however, 
the  prominent  masses  of  radial  storage  parenchyma  which  dis- 
tinguish the  groups  under  consideration  are  not  primarily  related 
to  the  appendages,  they  become  somewhat  definitely  limited  to 
this  position  in  many  forms,  and  even  in  the  Gnetales  themselves 
are  more  strongly  developed  in  connection  with  lateral  organs. 

We  may  next  turn 
to  the  anatomical 
organization  of  the 
genus  Gneium.  Fig.  261 
illustrates  the  general 
structure  of  the  stem 
in  this  genus  as  exem- 
plified by  a  young  stem 
of  Gnetum  scandens. 
The  appearance  is  very 
similar  to  that  of  a 
dicotyledonous  vine 
like  Clematis  or  Vitis. 
Extremely  prominent 
large  rays  separate  the 
woody  cylinder  into 
distinct  fibro vascular 
strands.  The  large  rays  of  Gnetum  are  distinguished  from  the  corre- 
sponding features  of  organization  in  Ephedra  by  two  important 
details.  In  the  first  place,  like  those  of  the  dicotyledonous  climbers 
cited  above,  they  extend  broadly  to  the  pith  and  do  not  appear  first 
as  narrow  rudiments  which  are  widened  as  they  pass  toward  the  out- 
side of  the  woody  cylinder.  Secondly,  the  broad  rays  of  Gnetum 
are  in  the  stem  usually  entirely  homogeneous;  that  is,  they  contain 
no  distinct  vestiges  of  fibrous  and  vascular  structures  such  as 
appear  in  the  broad  radial  storage  bands  of  Ephedra,  although 
in  types  like  G.  scandens  the  broad  rays  of  the  stem  are  not  obviously 
derived  from  compounding  of  aggregations  of  rays  and  longitudinal 
elements  of  the  wood.  Investigations  on  the  part  of  Professor 
W.  P.  Thompson  as  yet  unpublished  seem  to  make  it  clear,  how- 


Fio.  261. — Transverse  section  of  young  stem  of 
Gnetum  scandens. 


364 


THE  ANATOMY  OF  WOODY  PLANTS 


ever,  on  the  basis  of  the  organization  of  primitive  organs  and 
regions  that  the  condition  of  aggregation  presented  by  the  genus 
Ephedra  was  once  present  in  Gnetum.  We  may  therefore  regard 
the  large  rays  of  the  genus  as  comparable  with  the  similar  struc- 
tures in  the  mature  wood  of  the  stem  of  our  northern  oaks  and 
derived  in  a  similar  manner  as  the  result  of  aggregation  and  fusion. 
The  minute  organization  of  the  secondary  xylem  in  Gnetum  reveals 

in  addition  to  the 
prominent  rays, 
which  have  been  dis- 
cussed in  the  pre- 
ceding paragraph, 
other  radial  struc- 
tures which  are  of  the 
uniseriate  type  and 
correspond  with  the 
similar  rays  in  the 
Coniferales  and  with 
the  linear  rays  of  the 
wood  of  the  oak  and 
allied  dicotyledonous 
types.  Parenchyma 
is  likewise  abundantly 
present  in  the  diffuse 
condition.  Last  and  by  no  means  least,  vessels  which  in  most 
species  of  the  genus  present  the  large  caliber  characteristic  of 
climbers  are  present.  These  will  be  considered  later  in  a  special 
paragraph. 

In  Fig.  262  is  shown  an  older  stem  of  G.  scandens.  Here  the 
woody  cylinder  instead  of  consisting  of  a  single  circle  of  bundles 
has  become  polydesmic.  This  condition  cannot  be  regarded  as 
having  any  large  evolutionary  significance,  as  it  is  commonly 
found  in  the  stems  of  climbers  of  remote  systematic  affinities. 
Its  chief  significance  is  in  connection  with  the  origin  of  the  type 
of  cylinder  presented  by  the  genus  Welwitschia  and  that  found 
in  certain  cycadean  types,  living  and  extinct.  Gnetum  shows, 
not  only  the  polydesmic  stem  often  found  in  woody  climbers, 


FIG.  262.— Transverse  section  of  an  older  stem  of 
the  same. 


GNETALES  365 

but  also  the  extreme  condition  found  in  strap-shaped  lianae. 
Fig.  263  portrays  this  condition  in  the  stem  of  G.  latifolium.  The 
woody  strands  of  the  polycyclic  cylinder  fail  to  develop  on  two 
opposite  sides  of  the  axis,  and  this  organization  is  correlated  with 
the  flattened  transverse  section  in  the  stem.  It  is  obvious  that 
the  genus  under  discussion  has  advanced  to  a  condition  of 
organization  such  as  is  paralleled  by  perennial  climbers  among 
the  dicotyledons,  and  it  must  therefore  be  considered  as  having 
reached  a  high  evolutionary  position. 

The  broad  short  axis  of  the  remarkable  genus  Welwitschia  may 
now  claim  our  attention.     Here  the  stem  never  attains  a  height 


FIG.  263. — Transverse  section  of  the  flattened  stem  of  Gnetum  latifolium 

of  more  than  a  half-meter  and  bears  two  large  perennial  leaves 
which,  according  to  the  investigations  of  Bower,  are  not  the 
persistent  cotyledons,  but  a  subsequent  pair  of  foliar  organs.  The 
perennial  leaves  of  Welwitschia  are  inclosed  at  their  bases  in  hollow 
spaces  resulting  from  the  outgrowth  of  the  stem.  Within  these 
cavities,  which  function  as  moist  chambers,  are  situated  the  basal 
growing  regions  of  the  leaves;  they  are  thus  preserved  from  fatal 
desiccation  under  the  extremely  arid  conditions  connected  with 
the  desert  habitat  of  the  genus.  Professor  W.  P.  Thompson  has 
been  able,  through  the  co-operation  of  the  Sheldon  Foundation 
of  Harvard  University,  to  secure  an  abundant  supply  of  material 
illustrating  the  anatomical  organization  of  Welwitschia.  When 
the  results  of  his  investigations  have  been  published,  our  knowledge 
of  this  interesting  and  aberrant  South  African  genus  will  be  largely 
increased  beyond  that  supplied  in  Hooker's  well-known  memoir. 
The  general  organization  of  the  axis  in  the  genus  is  well  illustrated 


366 


THE  ANATOMY  OF  WOODY  PLANTS 


by  the  accompanying  figure  (Fig.  264)  of  the  stem  of  a  seedling 
collected  by  Professor  Thompson  in  Southwest  Africa.  It  is  clear 
that  the  same  polydesmic  organization  of  the  axis  is  present  as 
is  found  in  the  older  stem  of  species  of  Gnetum.  In  the  South 
African  genus,  however,  the  polydesmic  condition  extends  to  the 
roots  and  is  accordingly  to  be  regarded  as  a  more  innate  feature 
of  organization  than  in  Gnetum.  The  structure  of  Welwitschia 

suggests  a  climbing 
ancestry.  Although 
we  do  not  know  the 
explanation  of  the 
polydesmic  condition 
in  climbers,  it  is 
clearly  co-ordinated 
with  the  vine  habit. 
We  may  suppose  that 
the  forbears  of  the 
genus  were  originally 
forest  climbers  and 
that  the  surviving 
strongly  truncated 
desert  species  has  per- 
sisted in  its  present 
habitat  with  the  re- 
tention of  the  ances- 
tral polydesmy.  A  similar  suggestion  has  been  made  in  an  earlier 
chapter  as  a  possible  explanation  of  the  phenomenon  of  polydesmy 
in  the  cycadean  forms,  living  and  extinct. 

Having  considered  the  general  topography  of  the  stem  in  the 
Gnetales  with  particular  reference  to  the  presence  of  large  rays 
of  the  dicotyledonous  type  and  the  phenomenon  of  polydesmy,  we 
may  now  profitably  turn  our  attention  to  the  more  minute  organ- 
ization of  the  wood.  The  ligneous  tissues  consist  of  rays  and 
longitudinal  elements.  The  former  have  already  been  sufficiently 
discussed  in  previous  paragraphs.  The  longitudinal  structures 
of  the  xylem  consist  of  tracheids,  vessels,  and  storage  parenchyma. 
The  tracheids  do  not  need  any  extended  reference,  as  they  have 


FIG.  264.— Transverse  section  of  young  stem  of 
Welwitschia  mirabilis. 


GNETALES  367 

been  described  in  an  earlier  page  of  the  present  work.  They 
possess  clearly  marked  bordered  pits  larger  than  those  of  the 
dicotyledons  and  provided  with  a  distinct  torus.  The  vessels 
of  the  tribe  are  of  considerable  interest  from  the  standpoint  of 
the  doctrine  of  descent,  as  they  clearly  indicate  the  derivation  of 
vascular  structures  from  tracheids.  This  takes  place  by  the 
modification  of  the  terminal  regions  of  the  incipient  vessel  from 
gradually  tapering  to  distinctly  inclined  walls  at  angles  with  the 
sides  of  the  element.  These  differentiated  terminal  aspects  of 
the  vessels  are  provided  with  very  much  larger  pits  than  are  found 
in  the  lateral  walls.  These  pits,  however,  generally  in  Ephedra, 
and  apparently  universally  in  the  two  higher  genera,  lose  their 
membranes  at  an  early  stage,  and  free  intercommunication  is 
thus  established.  In  the  higher  genera  there  is  a  marked  tendency 
for  the  terminal  walls  of  the  vessel  to  develop  a  single  huge  bordered 
pit  in  which  the  membrane  is  lacking.  In  Ephedra,  on  the  other 
hand,  the  terminal  pits  are  numerous,  and  in  a  few  cases  are 
found  to  fuse  with  one  another  horizontally  with  the  resultant  ap- 
pearance of  slits  comparable  to  those  in  the  vessels  of  the  lower 
dicotyledons.  The  type  of  vessel  found  in  Ephedra  has  been  re- 
cently stated  to  persist  in  the  reproductive  axes  of  the  genus  Gnetum. 
The  parenchymatous  structures  of  the  Gnetales  need  no  extended 
reference,  for  on  the  whole  they  resemble  those  of  the  higher  Coni- 
ferales,  both  in  their  distribution  in  the  annual  ring  (and  this  is 
diffuse)  and  in  their  configuration.  Sometimes  structures  occur  in 
the  woods  of  the  Gnetales  more  or  less  resembling  substitute  fibers, 
since  with  pointed  elongated  configuration  they  unite  a  persistence 
of  protoplasmic  contents.  It  is  clear  from  the  brief  summary  of 
the  organization  of  the  wood  here  supplied  that  the  Gnetales  are 
of  great  importance  from  the  evolutionary  standpoint,  particularly 
in  connection  with  the  important  problems  presented  by  the 
evolution  of  large  rays  and  vessels.  They  furnish  a  valuable 
criterion  for  the  estimation  of  primitive  anatomical  characteristics 
in  the  organization  of  the  wood  in  that  huge  heterogeneous  aggre- 
gation of  forms  assembled  under  the  caption  of  dicotyledons. 
Their  value  in  this  respect  will  appear  at  a  later  stage.  Although 
the  Gnetales  clearly  indicate  conditions  of  anatomical  organization 


368  THE  ANATOMY  OF  WOODY  PLANTS 

which  are  of  a  primitive  nature,  we  are  unfortunately  even  less 
well  informed  as  to  their  fossil  representatives  than  in  the  case 
of  the  angiosperms.  There  are,  in  fact,  no  well-authenticated 
gnetalian  remains,  even  from  the  later  period  of  the  Mesozoic, 
in  which  the  dicotyledons  had  become  well  established  as  an 
important  component  of  the  plant  population  of  the  earth.  The 
wide  geographical  distribution  of  the  three  living  genera  may 
perhaps  be  regarded,  in  conjunction  with  their  anatomical  organ- 
ization, as  a  definite  indication  of  their  earlier  more  abundant 
occurrence. 

The  root  in  the  Gnetales  needs  no  special  consideration  in  the 
present  connection.  In  the  genus  Gnetum  it  furnishes  some 
evidence  as  to  the  original  organization  of  the  large  rays,  but  in 
Welwitschia  it  is  polydesmic  like  the  stem  and  is  of  little  value  from 
the  standpoint  of  evolutionary  anatomy.  The  structure  of  the 
root  in  Ephedra  closely  resembles  that  in  the  stem,  except  as 
regards  those  general  features  which  distinguish  root  from  axis. 

The  foliar  organs  of  the  group  which  forms  the  subject  of  the 
present  chapter  are  distinctly  gymnospermous  in  their  anatomy 
and  frequently  exhibit  the  copious  development  of  transfusion 
tissues  of  the  type  characteristic  of  the  leaves  of  the  Ginkgoales 
and  Coniferales.  Centripetal  wood  is  conspicuously  absent  in  the 
leaves  of  the  Gnetales,  unless  it  be  assumed  that  the  transfusion 
elements  are  actually  representatives  and  not  merely  derivatives 
of  the  old  centripetal  or  cryptogamic  wood.  On  account  of  the 
small  size  of  the  leaves  in  Ephedra  the  transfusion  tissues  are 
relatively  poorly  developed.  In  Gnetum  a  higher  systematic 
position  is  strongly  vouched  for  by  the  organization  of  the  leaf, 
in  which  transfusion  elements  are  not  particularly  well  developed. 
The  foliar  organ  of  Welwitschia  supplies  us  with  transfusionary 
features  most  strongly  manifested.  The  truth  of  this  statement 
may  be  verified  by  reference  to  Fig.  265,  which  illustrates  a  part 
of  a  transverse  section  of  the  leaf  of  the  South  African  genus. 
Transfusion  elements  originating  on  the  flanks  of  the  xylem  extend, 
as  in  certain  Coniferales,  above  and  below  outside  the  sclerenchyma- 
tous  sheath  which  surrounds  the  fibrovascular  bundles  of  the 
leaves. 


GNETALES 


369 


The  anatomy  of  the  Gnetales  is  of  particular  importance  at 
the  present  time  when  they  have  come  to  the  front  once  more, 
either  as  a  cognate  stock  with  the  dicotyledons  or  as  their  actual 
ancestors.  The  study  of  the  internal  organization  of  the  group 
in  comparison  with  the  dicotyledonous  angiosperms  reveals  many 
features  of  marked 
resemblance.  Both 
are  provided  with 
large  rays  which 
are  clearly  fusion 
products;  and  in 
both  the  wood 
shows  conducting 
elements  belonging 
to  the  category  of 
vessels.  The  rays 
apparently  supply 
a  very  cogent  argu- 
ment for  the  close 
affinity  of  the 
Gnetales  and  the 
angiosperms.  I  n 
the  case  of  the 
vascular  struc- 
tures, however,  it 
is  not  so  clear  that 
a  morphological 
identity  of  the  ele- 
ments present  in 
the  two  groups  can  be  successfully  maintained.  In  the  dicotyledons 
the  pits  present  on  the  terminal  walls  of  the  vascular  elements  are 
not  larger  than  those  which  appear  laterally,  and  the  perforation 
of  the  vessel  takes  place  as  the  result  of  fusions  of  opposite  pits,  as 
has  been  shown  in  an  earlier  chapter.  A  very  different  situation 
manifests  itself,  as  indicated  above,  in  the  genera  of  the  Gnetales. 
Here  the  vascular  elements  have  exceedingly  large  pits  on  their 
terminal  walls,  and  in  Ephedra  these  are  usually  without  membranes, 


FIG.  265. — Transverse  section  of  a  leaf  bundle  in 

Welwitschia  mirabilis. 


370  THE  ANATOMY  OF  WOODY  PLANTS 

while  in  the  other  two  genera  they  are  invariably  so  characterized. 
The  phenomenon  of  fusion  of  the  pits  to  form  the  scalariform 
perforations  present  in  the  lowest  type  of  vessel  of  the  dicoty- 
ledons is  rare  in  the  woods  of  the  Gnetales.  The  general 
anatomical  organization  of  the  fibrovascular  tissues  in  the  Gnetales, 
however,  may  be  said  distinctly  to  favor  the  hypothesis  of  dicoty- 
ledonous affinities,  or,  at  any  rate,  to  be  more  in  harmony  with 
such  relationship  than  with  that  of  any  other  group  of  seed  plants. 
The  affinities  of  the  Gnetales  on  the  gymnospermous  side  are 
much  more  difficult  to  discover.  This  situation  is  due  no  doubt 
in  large  part  to  the  fact  that  as  yet  no  properly  authenticated 
fossil  representatives  of  the  group  have  been  found.  In  recent 
years  there  has  been  a  tendency  to  associate  this  group  with  the 
Cycadales,  but  it  is  extremely  difficult  to  discover  any  valid 
reasons  from  the  anatomical  side  to  justify  such  a  relationship. 
There  seem,  in  fact,  to  be  no  anatomical  features  of  wide  validity 
which  can  be  invoked  in  favor  of  an  affinity  between  the  highest 
living  gymnosperms  and  the  lowest.  It  seems  on  the  whole 
more  probable  that  the  Gnetales  are  cognate  with,  or  derived 
from,  the  Coniferales,  since  there  are  a  number  of  features  which 
make  such  a  connection  likely.  The  transfusion  tissue  of  the 
highest  gymnosperms  is  clearly  of  the  coniferous  type,  and  this 
feature  must  count  strongly  against  any  close  relationship  with 
the  Cycadales,  in  which,  owing  to  the  strong  persistence  of  the 
centripetal  wood,  true  transfusion  tissues  have  not  yet  made  their 
appearance.  The  uniseriate  rays  of  Ephedra  and  Gnetum  also 
point  to  coniferous  affinities  and  not  toward  a  connection  with 
the  Cycadales,  in  which  the  rays  are  universally  multiseriate. 
The  presence  of  bars  of  Sanio  in  the  vessels  of  Ephedra  supplies 
likewise  an  argument  for  the  coniferous  rather  than  the  cycadalian 
relationship. 

The  main  support  for  the  cycadean  origin  of  the  gymnospermous 
group  which  forms  the  subject  of  the  present  chapter  has  been 
derived  from  the  reproductive  structures  of  the  living  and  extinct 
representatives  of  the  cycadean  stock.  The  investigations  of 
Wieland  have  brought  to  our  knowledge  the  complete  organization 
of  the  reproductive  parts  in  the  Bennetti tales.  These  consist 


GNETALES  371 

of  cones  involved  in  the  young  condition  in  rather  large  sheltering 
bracts.  Immediately  above  the  series  of  bracts  microsporophylls 
(usually  of  large  size  and  pinnate  structure)  are  attached  to  the 
axis  of  the  strobilus.  The  main  part  of  the  cone  is,  however, 
made  up  of  the  seminal  organs,  consisting  of  pedicels  to  which 
single  seeds  are  attached  in  an  orthotropous  manner.  The  seeds 
are  somewhat  sheltered  by  the  swollen  ends  of  sterile  appendages 
inserted  among  the  seminal  organs  on  the  axis.  The  protected 
condition  of  the  seeds  has  given  rise  to  the  suggestion  of  angio- 
spermous  organization.  This  apparently  cannot  be  regarded  as 
more  than  the  merest  analogy,  since  the  seeds  are  not  shel- 
tered within  a  closed  megasporophyll  and  the  pollen  is  deposited 
directly  upon  them  after  the  typical  gymnospermous  manner. 
Further,  the  cycadean  fertilization  is  not  even  siphonogamous, 
as  would  be  expected  of  a  group  presenting  valid  claims  to  be 
regarded  as  ancestral  to  the  highest  seed  plants.  Nor  is  the 
analogy  with  the  flower  of  the  angiosperms  to  be  given  greater 
weight,  for  the  arrangement  alone  and  not  the  intimate  organiza- 
tion of  the  reproductive  parts  shows  any  real  resemblance  to 
the  conditions  found  in  the  floral  structures  of  the  angiosperms. 
An  androgynous  cone  such  as  is  frequently  present  ^n  the  Coniferales 
supplies  an  equally  good  basis  for  comparison;  for  here  the  bracts 
below  correspond  to  the  floral  envelopes  of  the  flower,  while  the 
lower  whorls  of  microsporophylls  simulate  the  anthers,  and  a 
plausible  resemblance  to  the  carpellary  whorls  is  presented  by  the 
ovuliferous  scales  of  the  upper  region  of  the  modified  cone.  The 
most  striking  objection  and  the  same  as  occurs  in  the  case  of  the 
Bennettitales  is  the  fact  that  the  ovuliferous  scales  are  not  angio- 
spermous.  The  siphonogamic  fertilization  of  the  Araucarian  coni- 
fers, on  the  other  hand,  reveals  a  nearer  degree  of  resemblance  to 
the  conditions  found  in  the  flower  of  the  angiosperms  than  does 
the  zoidogamy  of  the  archigymnospermous  Bennettitales.  In  the 
present  state  of  our  ignorance  of  the  fossil  ancestors  of  the  Angio- 
spermae  it  seems  impossible  to  fix  on  any  group  of  gymnosperms, 
living  or  extinct,  except  the  Gnetales,  which  can  be  regarded  with 
any  degree  of  probability  as  having  been  either  ancestral  to  the 
highest  of  the  seed  plants  or  even  cognate  with  them. 


372  THE  ANATOMY  OF  WOODY  PLANTS 

In  accordance  with  the  hypothesis  of  the  derivation  of  the 
angiosperms  from  the  Bennettitales,  a  relationship  between  this 
cycadean  group  and  the  Gnetales  has  been  suggested.  There 
seem  to  be  very  slight  grounds  for  this  assumption  of  affinities. 
The  so-called  "pollen  chamber"  of  Ephedra  is  not  morphologically 
equivalent  to  the  pollen  chamber  of  the  lower  gymnosperms 
which  is  derived  by  the  breaking  down  of  cells  in  the  lysigenous 
manner,  and  in  this  respect  is  in  marked  contrast  to  the  depression 
on  the  apex  of  the  nucellus  of  Ephedra,  which  is  continuously 
covered  with  the  epidermis.  Moreover,  Ephedra  and  the  other 
Gnetales  are  siphonogamous  in  contrast  to  the  zoidogamous 
condition  of  fertilization  in  the  Cycadales.  The  details  of  organ- 
ization of  such  a  flower-like  structure  as  is  found  in  Welwitschia 
apparently  supplies  no  adequate  basis  for  comparison  with  the 
bisporangiate  strobilus  of  the  Bennettitales.  The  original  error  of 
Saporta  in  designating  the  impressions  of  the  fructifications  of 
bennettitean  types  as  belonging  to  hypothetical  proangiosperms 
or  primitive  angiosperms  has  manifested  a  considerable  degree  of 
vitality  in  the  rather  long  interval  since  it  was  first  advanced; 
but  there  seems  little  reason  to  accept  it  at  the  present  time,  in 
view  of  our  increased  knowledge  of  the  organization  of  both  vege- 
tative and  reproductive  parts  in  the  various  groups  of  seed  plants, 
living  or  extinct. 

The  Gnetales  are  clearly  gymnosperms  which  in  certain  features 
of  anatomical  structure  and  reproductive  organization  have  made 
a  marked  advance  in  the  direction  of  the  characteristics  of  the 
angiosperms,  and  it  is  not  improbable  that  they  are  at  least  a 
cognate  group.  They  present,  however,  no  valid  resemblances 
in  either  their  reproductive  or  their  vegetative  parts  which  justify 
an  assumption  of  relationship,  even  remote,  with  the  extinct 
bennettitean  Cycadales.  It  therefore  appears  highly  unlikely 
that  either  the  angiosperms  or  the  Gnetales  have  any  close  degree 
of  relationship  with  any  archigymnospermous  group,  although 
it  seems  not  at  all  improbable  that  they  are  somewhat  closely 
related  to  one  another. 


CHAPTER  XXVI 
THE  ANGIOSPERMS 

The  angiosperms  as  a  whole  possess  features  which  separate 
them  sharply  both  reproductively  and  vegetatively  from  gymno- 
spermous  groups.  The  first  of  these  in  importance  is  the  phenom- 
enon of  angiospermy  itself.  In  the  group  the  microspores  or 
pollen  grains  no  longer  reach  the  micropylar  canal  at  the  apex  of 
the  ovule,  but  are  accommodated  on  the  tip  of  the  megasporophyll, 
which  becomes  modified  as  the  receptive  prominence  or  stigma. 
The  reception  of  the  microspores  on  the  terminal  region  of  the 
carpel  or  sporophyll  is  not,  however,  an  exclusive  characteristic 
of  the  angiosperms,  since  the  same  feature  is  presented  by  the 
surviving  araucarian  conifers.  The  megasporophylls  in  this 
largest  and  most  important  group  of  the  seed  plants  are  either 
folded  upwardly  upon  themselves  in  such  a  manner  as  to  inclose 
the  young  seeds  or  ovules,  or  undergo  protective  fusions  with 
other  similar  structures  in  the  same  flower.  The  angiosperms 
are  characterized  anatomically  throughout  by  the  possession  of 
vessels,  these  structures  being  absent  only  in  certain  aberrant 
representatives  of  the  Cactaceae,  Crassulaceae,  and  Trochoden- 
draceae.  In  all  the  exceptional  cases  mentioned  there  is  clear 
evidence  on  comparative  anatomical  and  experimental  grounds 
that  vessels  were  formerly  present.  The  possession  of  histological 
structures  in  the  wood  known  as  vessels  may  accordingly  be  set 
down  as  a  primitive  feature  of  organization  of  the  angiosperms. 
Associated  with  the  vascular  structures  just  indicated,  there  is  a 
general  improvement  in  the  storage  devices  of  the  wood  which, 
in  an  extreme  form,  leads  to  the  appearance  of  the  herbaceous  type. 
The  gametophytes,  particularly  the  male  gametophytes,  of  the 
angiosperms  present  a  very  marked  degree  of  uniformity  in  the 
two  great  divisions  of  the  group.  The  microspore  or  pollen  grain 
undergoes  part  of  its  germination  in  the  microsporangium  and 
develops  normally  a  tube  nucleus  and  a  generative  cell.  The 

373 


374  THE  ANATOMY  OF  WOODY  PLANTS 

latter,  unlike  the  similar  structure  in  the  microspores  of  the  higher 
Coniferales,  does  not  give  rise  to  stalk  and  body  cells,  but  originates 
directly  by  division  the  two  generative  nuclei  which  function  in 
fertilization.  The  microspores  are  sheltered  in  sporangia,  which 
are  typically  four  in  number,  on  each  sporophyll.  The  sporangia 
owe  their  dehiscence  to  a  mechanical  layer  within  the  wall,  resem- 
bling in  structure  that  found  in  Ginkgo  and  certain  Coniferales,  but 
no  longer  related,  as  in  these,  to  the  fibrovascular  system  of  the 
sporophyll.  The  megasporangium  is  much  modified  and,  as  in 
other  known  seed  plants,  is  without  any  opening  mechanism. 
In  certain  of  the  lower  and  amentiferous  dicotyledons,  however, 
tracheary  structures  have  been  discovered  in  the  inferior  region 
of  the  megasporangium  (Casuarina,  Corylus,  Castaned).  The 
germinated  megaspore  normally  gives  rise  to  a  gametophyte 
containing  originally  eight  naked  nuclei  which  result  from  three 
successive  divisions.  Of  these,  six  become  organized  as  cells  by 
the  development  of  inclosing  protoplasmic  bodies,  while  the  two 
remaining  nuclei,  one  from  each  pole  of  the  elongated  sac,  unite 
to  constitute  the  endosperm  nucleus,  a  structure  characteristic 
for  the  angiosperms  and  not  occurring  in  any  lower  forms.  Three 
cells  in  the  micropylar  region  of  the  gametophyte  become  organized 
as  the  single  egg  and  the  two  synergidae.  Three  others  constitute 
the  antipodals  in  the  base  of  the  gametophyte.  After  the  egg  has 
been  fertilized  by  one  of  the  sperm  nuclei  of  the  pollen  tube  and 
the  endosperm  nucleus  has  contracted  a  union  with  the  remaining 
male  element,  the  egg  develops  as  the  embryo  and  the  endosperm 
nucleus  gives  rise  to  a  mass  of  tissue  which  usually  quite  supplants 
the  original  gametophyte  or  embryo  sac  as  nourishing  substance 
for  the  developing  embryo.  The  seed  of  the  angiosperms  conse- 
quently typically  contains,  in  addition  to  the  gametophyte  and 
sporophyte  of  lower  seed  plants,  a  third  generation.  This  is 
known  as  the  trophophyte  or  endosperm,  and  it  is  distinguished 
from  gametophyte  and  sporophyte,  not  only  by  its  peculiar  mode 
of  origin,  but  also  by  the  fact  that  in  its  nuclear  divisions  three 
times  as  many  chromosomes  are  present  as  in  the  gametophyte 
and  one-half  more  than  in  the  sporophyte.  This  cytological 
condition  is  doubtless  due  to  the  three  nuclei  which  are  fused  to 


THE  ANGIOSPERMS  375 

constitute  the  original  endosperm  nucleus  from  which  the  cells 
of  the  endosperm  or  trophophyte  take  their  origin.  The  high 
degree  of  constancy  in  the  essential  features  of  organization  of 
both  sporophyte  and  gametophyte  in  the  angiosperms  mark  them 
as  a  monophyletic  group  in  which  the  two  great  divisions,  dicoty- 
ledons and  monocotyledons,  have  had  a  common  origin. 

There  seems  to  be  no  reasonable  doubt  that  both  divisions  of 
the  angiosperms — the  dicotyledons  and  monocotyledons — have 
originated  from  gymnospermous  ancestors  and  not  directly  from 
any  existing  or  extinct  group  of  vascular  cryptogams,  since  they 
entirely  lack  cryptogamic  features  of  organization  in  all  their 
organs  with  the  sole  exception  of  the  root.  The  radical  organ, 
however,  is  without  significance  as  indicating  cryptogamic  deriva- 
tion, for  cryptogamic  or  centripetal  primary  wood  is  present  in 
all  roots  without  exception.  It  further  seems  obvious  that  the 
angiosperms  in  neither  of  their  two  divisions  can  have  originated 
from  the  Archigymnospermae,  since  they  present  the  siphonoga- 
mous  mode  of  fertilization  in  contrast  to  the  zoidogamy  or  fertiliza- 
tion by  antherozoids  present  in  the  lower  gymnospermous  tribes 
which  are  more  nearly  related  to  the  Filicales. 

The  general  features  of  the  angiosperms  indicated  above 
characterize  the  two  great  divisions,  dicotyledons  and  monocotyle- 
dons, in  common;  and  it  is  now  necessary  to  specify  the  distinguish- 
ing structures  which  separate  these  from  one  another.  Since  the 
dicotyledons  are  with  the  greater  probability  the  older  and  more 
primitive  of  the  two  main  groups,  they  will  first  be  considered. 
In  the  dicotyledonous  angiosperms  the  seed  is  distinguished  by 
an  embryo  possessing  paired  cotyledons  or  seed  leaves.  This 
feature  is  perhaps  the  most  constant  characteristic  of  the  group. 
In  the  fibrovascular  structures  the  wood  is  distinguished  by  well- 
marked  secondary  growth  which  becomes  feeble  only  in  forms  in 
which  the  herbaceous  habit  has  become  distinctly  developed.  The 
tissues  belonging  to  the  conductive  category  are,  moreover,  typi- 
cally arranged  in  the  form  of  a  cylinder  which  is  continuous  in 
woody  forms  but  becomes  broken  up  into  separate  strands  in 
stems  with  herbaceous  texture.  When  the  fibrovascular  organiza- 
tion consists  of  isolated  bundles,  these  are  ordinarily  arranged  in 


376  THE  ANATOMY  OF  WOODY  PLANTS 

a  circular  fashion,  but  occasionally  in  stems  with  large  leaves 
provided  with  numerous  foliar  traces  the  periphery  of  the  cylinder 
is  no  longer  capable  of  accommodating  the  bundles,  so  they  have 
to  be  disposed  of  in  a  position  either  medullary  or  cortical.  The 
leaves  of  the  dicotyledons  are  usually  distinguished  by  skeletal 
structures  or  veins  ending  freely  toward  the  margins.  The  main 
veins  may  be  arranged  either  in  a  radiating  or  in  a  palmate  fashion, 
or  may  take  their  origin  in  an  alternating  manner  from  opposite 
sides  of  a  main  vein  or  midrib,  in  which  case  the  venation  is  said 
to  be  pinnate.  The  free  venation  of  dicotyledonous  angiosperms 
gives  them  a  considerable  advantage  over  the  monocotyledons 
in  the  possibility  of  submerged  existence  or  a  shaded  habitat,  when 
their  leaves  often  become  finely  dissected.  The  root  of  dicotyledons 
presents  no  special  features  worthy  of  note  in  a  general  statement. 
The  floral  parts  are  ordinarily  present  in  multiples  of  five,  and 
the  floral  envelopes  show  themselves  on  the  whole  less  likely  to 
vary  from  the  pentamerous  condition  than  do  the  essential  or 
strictly  reproductive  whorls,  the  stamens  and  pistils.  Pollination 
is  sometimes  effected  through  the  agency  of  currents  in  the  air, 
but  more  commonly  in  the  higher  families  by  insects.  Fertiliza- 
tion results  from  the  penetration  of  the  pollen  tube  from  the  stigma 
to  the  micropyle  of  the  ovule.  The  course  of  the  pollen  tube 
after  it  leaves  the  region  of  the  style  may  either  be  direct  through 
the  cavity  of  the  ovary  to  the  micropyle  or,  avoiding  the  leap 
across  the  ovarial  air  space,  it  may  make  its  way  round  through 
the  basal  or  chalazal  region  of  the  ovule.  The  last  method  of 
fertilization  is  characteristic  of  the  genus  Casiiarina,  the  Betu- 
laceae,  the  Juglandaceae,  certain  Urticaceae,  etc.,  and  is  known 
as  breech  fertilization  or  chalazogamy.  It  has  been  suggested 
by  Treub  and  Nawaschin  that  this  is  a  primitive  method  of  pene- 
tration for  the  angiosperms  and  marks  a  transition  from  the 
siphonogamous  and  higher  gymnosperms  in  which  the  pollen, 
being  deposited  directly  upon  the  ovule,  has  not  become  accustomed 
to  leaping  an  air  space.  In  the  lower  representatives  of  the 
dicotyledons  the  pollen,  although  no  longer  deposited  on  the 
micropyle,  still  maintains  its  primitive  inability  to  cross  an  air 
space.  There  is  much  to  be  said  for  the  hypothesis  of  the  primitive 


THE  ANGIOSPERMS 


377 


significance  of  chalazogamy  from  the  standpoint  of  fibrovascular 
anatomy.  It  seems,  however,  a  feature  too  likely  to  be  modified 
somewhat  rapidly  by  conditions  to  rank  as  a  criterion  of  the  first 
order  for  the  distinguishing  of  the  most  primitive  dicotyledons. 

The  monocotyledons  are  distinguished,  as  their  name  indicates, 
by  a  seed  containing  an  embryo  with  a  single  cotyledon  or  seed 
leaf.  This  feature  is  very  constant,  but  there  are  indications  of 
the  presence  of  a  second  cotyledon  in  certain  of  the  grasses,  such 
as  Zizania,  Avena,  etc.  The  fibrovascular  strands  of  this  group 
are  ordinarily  closed;  that  is,  they  do  not  possess  the  capacity  to 
increase  in  thickness  through  the  activity  of  a  cambial  layer. 
The  arrangement  of  the  strands  in  the  monocotyledonous  angio- 
sperms  is  also  distinctive,  since  the  bundles,  instead  of  being 
disposed  in  a  circular  fashion  as  in  the  mass  of  herbaceous  dicotyle- 
dons, are  scattered  throughout  the  transverse  section  of  the  cylinder 
and  sometimes  even  occur  in  the  cortex.  This  peculiar  disposition 
is,  beyond  any  reasonable  doubt,  the  result  of  the  entrance  of 
numerous  leaf  traces  into  the  axis  at  each  node,  a  consequence  of 
the  high  assimilative  efficiency  of  the  foliar  organs.  The  veins 
of  monocotyledonous  leaves  are  distinguished  primitively  by  a 
closed  arrangement;  that  is,  starting  out  at  the  base  of  the  leaf 
as  a  closed  system,  they  reunite  at  the  apex  of  the  leaf.  This 
disposition  of  the  skeletal  tissues  of  the  leaf  makes  it  immune 
from  tearing  action.  The  lateral  veins,  in  consequence  of  this 
situation,  are  largely  abortive.  In  many  palms,  aroids,  and 
Scitamineae  the  venation  of  the  leaf  becomes  open  as  a  result  of 
changes  in  the  apex.  In  such  cases  the  venation  of  the  early- 
formed  leaves  of  seedlings  is  closed,  showing  that  this  condition 
is  the  primitive  one  for  the  group.  The  root  in  monocotyledons 
is  distinguished,  as  is  the  stem,  by  the  absence  of  secondary  growth. 
The  bundles  are  usually  distributed  in  a  circular  and  radial  fashion, 
but  in  certain  palms  and  orchids  they  may  be  scattered  throughout 
the  transverse  section  of  the  organ  as  they  are  in  the  stem.  The 
floral  parts  occur  in  multiples  of  three,  and  the  floral  envelopes, 
as  in  the  dicotyledons,  show  less  numerical  variability  than  do 
the  essential  whorls  consisting  of  stamens  and  carpels.  Pollination 
is  effected  usually  through  the  agency  of  insects,  but  may  be 


378  THE  ANATOMY  OF  WOODY  PLANTS 

brought  about  by  currents  of  air  in  some  of  the  probably  more 
primitive  groups.  Fertilization  is  always  porogamous  (through 
the  micropyle),  and  chalazogamy  is  at  the  present  time  quite 
unknown  in  the  monocotyledons.  The  members  of  this  group 
are  extremely  important  as  food  plants  on  account  of  their  high 
efficiency  in  the  elaboration  of  assimilates.  The  proportion  of 
seed  produced  by  some  of  the  cereals  in  a  vegetation  period  of 
three  or  four  months  is  often  over  30  per  cent  of  the  total  weight — 
a  relative  productiveness  seldom  realized  in  other  plants. 


CHAPTER  XXVII 
THE  WOODY  DICOTYLEDONS 

As  a  matter  of  convenience  the  anatomical  organization  of  the 
woody  dicotyledons  will  be  considered  in  the  present  chapter. 
It  must  not  be  supposed,  however,  that  such  a  mode  of  treatment 
implies  that  the  woody  texture  of  the  stem  has  any  value  from  the 
phylogenetic  or  taxonornic  standpoint.  Perennial  dicotyledons 
are  distinguished  by  the  possession  of  a  thick  woody  cylinder 
resulting  from  the  activity  of  a  cambial  layer  situated  between 
phloem  and  xylem;  this  adds  largely  to  the  wood  and  less  copiously 
to  the  inner  bark  during  the  periods  of  growth.  The  woods  of  the 
dicotyledons  offer  a  great  range  of  structural  organization,  and 
their  identification  on  the  basis  of  anatomical  features  is  corre- 
spondingly difficult. 

In  all  except  a  few  instances  dicotyledonous  woods  are  pro- 
vided with  vessels.  These  belong  to  two  main  types,  namely, 
those  with  scalariform  and  those  with  porous  perforations.  It 
has  been  made  clear  in  an  earlier  chapter  dealing  with  the  structure 
and  organization  of  the  vessel  that  the  type  with  scalariform  per- 
foration of  the  inclined  terminal  walls  results  from  the  fusion  of 
rows  of  opposite  pits  which  gives  rise  to  elongated  horizontal 
pores  from  which  the  membranes  quickly  disappear.  This  process 
repeated  in  successive  rows  of  pits  results  in  the  appearance  of 
lattice-like  or  scalariform  perforations  in  the  ends  of  the  vessels, 
and  these  permit  a  ready  passage  of  water.  On  general  evolution- 
ary principles  the  vessel  with  scalariform  perforations  is  to  be 
regarded  as  more  primitive  than  the  porous  type  presently  to 
be  discussed.  It  is  not  surprising  for  that  reason  that  it  is  a 
characteristic  feature  of  groups  which  are  considered  on  good 
grounds  to  be  low  in  the  dicotyledonous  scale.  The  vessel  with 
the  porous  type  of  perforation  is  clearly  derived,  as  has  been  dem- 
onstrated in  an  earlier  chapter,  from  the  scalariform  condition, 
in  the  first  instance  at  any  rate,  by  the  loss  of  the  bars  of 

379 


380  THE  ANATOMY  OF  WOODY  PLANTS 

lattice-work  through  mucilaginous  degeneration.  The  vessel  of  the 
second  type  is  found  in  the  woods  of  the  higher  groups  and  indi- 
cates a  more  advanced  condition  of  evolution,  other  things  being 
equal,  than  does  the  vascular  type  with  scalariform  terminal 
perforations.  Frequently  woods  with  the  porously  perforated 
'type  of  vessel  in  their  mature  structure  show  the  scalariform  con- 
dition in  the  region  of  the  primary  wood,  thus  providing  confirma- 
tion of  the  conclusion  that  the  latticed  terminal  wall  of  the  vessel 
is  phylogenetically  older  than  that  in  which  large  pores  are  present. 
In  many  instances  vessels  in  dicotyledonous  woods  become 
more  or  less  degenerate  and  are  then  easily  recognized  by  their 
inclined  end  walls  and  in  any  case  by  a  lateral  pitting  and  internal 
sculpture  which  clearly  distinguishes  them  from  tracheids  or  fibers. 
Such  vessels  are  often  present  in  woods  in  which  the  fibrous  ele- 
ments are  of  the  nature  of  libriform,  substitute,  or  septate  fibers, 
and  are  frequently  inappropriately  designated  as  tracheids.  True 
tracheids  have  always  tapering  or  fusiform  ends  and  are  not 
provided  with  the  lateral  pitting  and  sculpture  of  vessels.  It  is 
important  to  diagnose  degenerate  vessels  as  such,  because  a  failure 
to  make  this  distinction  may  result  in  quite  erroneous  views  as 
to  the  relationship  of  woody  dicotyledonous  forms.  In  some 
cases  typical  vessels  may  disappear  altogether  from  the  mature 
structure,  either  generally  or  locally.  Cases  of  the  general  dis- 
appearance of  vascular  elements  are  supplied  by  certain  Cactaceae 
and  Crassulaceae.  In  the  genus  Drimys  among  the  Magnoliaceae 
and  in  certain  genera  of  the  allied  Trochodendraceae  vessels  have 
also  entirely  disappeared  from  the  normal  wood.  In  Drimys, 
interestingly  enough,  wounds,  especially  wounds  of  the  root, 
recall  elements  which  have  the  lateral  sculpture  of  normal  vessels 
of  the  Magnoliaceae,  without  manifesting,  however,  their  charac- 
teristic scalariform  perforations.  This  defect  is  easily  explained 
as  a  result  of  the  comparatively  small  size  of  the  reversionary 
elements  simulating  vessels.  Local  absence  of  vessels  is  frequently 
found  in  connection  with  the  development  of  the  large  rays  in 
dicotyledonous  woods.  This  is  particularly  well  illustrated  in 
the  wood  of  Alnus  and  in  the  root- wood  of  certain  of  our  northern 
oaks.  In  the  region  of  the  aggregate  ray,  as  has  been  indicated 


THE  WOODY  DICOTYLEDONS  381 

in  earlier  pages,  the  organization  includes  only  tracheary  elements. 
It  is  thus  apparent  that  vessels,  a  characteristic  feature  of  structure 
in  dicotyledonous  woods,  may  in  certain  instances  be  absent. 
There  is  no  reason  based  on  the  general  principles  of  comparative 
anatomy  for  regarding  their  absence  as  a  primitive  feature.  In 
woods  of  temperate  climates  the  vessels  in  the  spring  wood  are 
often  much  larger  than  in  the  later  growth,  and  the  organization 
in  such  cases  is  described  as  ring-porous.  The  ring-porous  condi- 
tion is  not,  however,  universal  in  trees  of  higher  latitudes  and  is 
not  ordinarily  found  in  tropical  woods  which  in  general  have  their 
annual  zones  indistinctly  marked. 

The  tracheary  elements  in  dicotyledonous  wood,  as  has  been 
indicated  previously,  undergo  very  numerous  modifications.  In 
the  lower  forms  they  resemble  the  similar  structures  hi  the  gym- 
nosperms,  but  quite  generally  they  lose  to  a  large  extent  their 
water-conducting  function  and  become  mechanical  or  storage 
elements.  The  least  advanced  condition  in  the  mechanical  direc- 
tion is  designated  the  fiber- tracheid,  distinguished  by  the  re- 
duction in  size  and  decrease  in  number  of  the  bordered  pits  as 
well  as  by  increase  in  length  and  by  thickening  of  the  walls.  The 
libriform  fiber,  a  further  modification,  has  lost  or  nearly  lost  the 
bordered  pits,  these  being  replaced  by  simple  pores.  In  the  sub- 
stitute fiber,  which  retains  its  protoplasmic  contents,  and  in  the 
septate  fiber,  which  is  divided  by  delicate  partitions  of  pectic  cel- 
lulose into  a  number  of  separate  units,  are  seen  storage  modifica- 
tions of  the  tracheids  present  in  the  higher  types  of  dicotyledonous 
woods. 

The  parenchymatous  elements  of  dicotyledons  are  primitively 
scattered  throughout  the  annual  ring.  Although  rarely  and  only 
under  experimental  conditions  revealing  by  actual  transitions 
derivation  from  tracheids,  the  storage  parenchyma  is  generally 
grouped  in  longitudinal  fusiform  or  pointed  series  with  robust 
and  lignified  partitions  which  clearly  indicate  its  origin.  In  the 
systematically  higher  dicotyledons  the  parenchyma  is  character- 
istically grouped  in  clusters  around  the  vessels.  This  condition  is 
known  as  vasicentric  and  is  a  common  feature  of  woods  in  which  the 
tracheary  elements  have  become  partially  or  entirely  mechanical. 


382  THE  ANATOMY  OF  WOODY  PLANTS 

It  must  not  be  supposed,  however,  that  the  relation  between  me- 
chanical fibrous  elements  and  vasicentric  parenchyma  is  abso- 
lute, for  in  groups  characterized  by  this  mode  of  parenchymatous 
distribution  it  is  present  even  in  genera  with  tracheary  mechanical 
cells.  In  other  words,  the  grouping  of  parenchyma  about  the 
vessels  has  a  deeper  significance  than  that  of  mere  convenience 
to  water  supply.  In  not  a  few  instances  the  storage  cells  may  be 
confined  to  the  end  of  the  annual  ring.  This  is,  for  example,  the 
situation  found  in  the  Salicales,  and  it  also  frequently  characterizes 
genera  of  the  Magnoliaceae  occurring  in  temperate  climates.  In 
these  instances  an  examination  of  the  primitive  regions,  together 
with  experimental  investigation,  reveals  as  the  original  condition 
either  the  vasicentric  or  the  diffuse  distribution  of  parenchyma. 
It  may  be  stated  summarily  that  diffuse  storage  elements  constitute 
the  primitive  conditions  in  the  woods  of  the  dicotyledons,  and 
that  a  later  modification  is  the  vasicentric.  By  reduction  either 
of  the  two  types  mentioned  may  give  rise  to  the  terminal  condi- 
tion. Terminal  parenchyma  is  accordingly  a  phenomenon  of 
reduction  in  the  dicotyledonous  series,  while  in  the  Coniferales, 
as  has  been  elucidated  at  an  earlier  stage,  it  represents  the  primitive 
state  in  which  all  transitions  between  tracheary  and  storage  ele- 
ments are  frequently  and  normally  found. 

It  is  in  the  organization  of  their  wood  rays  or  radial 
storage  tissues  that  the  dicotyledons  manifest  the  most  distinct 
differences  from  the  mass  of  the  gymnosperms.  It  has  been  made 
clear  in  previous  pages  that  the  primitive  type  of  ray  organization 
for  the  group  was  the  linear  or  uniseriate  ray.  In  the  earliest 
conditions  presented  to  our  investigation,  however,  that  is  ac- 
companied by  the  aggregate  ray,  consisting  of  more  or  less  fused 
congeries  or  clusters  of  rays  separated  by  fibrous  elements.  This 
type  we  must  regard  as  an  original  one  for  rays  other  than  unise- 
riate in  the  dicotyledons,  because  it  is  clearly  found  in  Ephedra, 
by  common  consent  the  most  primitive  representative  of  the 
Gnetales,  and  because  it  is  extremely  persistent  in  primitive 
organs  and  regions  in  the  dicotyledons  themselves.  The  facility 
with  which  fibrous  elements  are  transformed  into  storage  cells 
in  the  group  under  consideration  has  led  to  the  metamorphosis 


THE  WOODY  DICOTYLEDONS  383 

of  the  aggregate  ray,  described  above,  into  large  homogeneous 
masses  of  radial  parenchyma  as  a  consequence  of  the  parenchyma- 
tous  modification  of  the  separating  fibers.  This  condition,  known 
as  the  compound  ray,  is  found  in  relatively  few  dicotyledonous 
woods,  and  these  are  ordinarily  regarded  as  low  in  the  systematic 
scale.  In  woody  types,  moreover,  it  very  readily  passes  into  the 
antecedent  aggregate  condition.  A  third  and  the  commonest 
condition  of  organization  of  the  radial  parenchyma  in  dicoty- 
ledonous woods  is  presented  by  the  diffuse  condition.  Here  the 
original  aggregation,  instead  of  retaining  its  identity  or  passing 
into  the  compound  state  last  described,  spreads  out  in  the  manner 
of  a  fan.  This  procedure  results  in  the  diffusion  of  the  original 
aggregations  of  rays  evenly  throughout  the  structure  of  the  wood. 
As  a  consequence  of  this  phenomenon  the  organization  of  the 
wood  in  the  mass  of  dicotyledons  is  characterized  by  the  presence 
of  abundance  of  rays  which  are  of  mediocre  width.  Sometimes 
the  rays  of  this  type  are  nearly  equal  in  size,  but  very  generally 
they  range  from  extremely  small  to  moderately  large.  Now  and 
then,  however,  as,  for  example,  in  the  wood  of  beeches  of  the  North- 
ern Hemisphere  and  in  the  genus  Platanus,  extremely  large  rays 
are  found,  readily  distinguishable  from  those  of  the  oak  type 
by  the  fact  that  they  grade  almost  imperceptibly  into  radial 
parenchymatous  bands  of  mediocre  dimensions.  In  the  typical 
compound  ray  such  as  occurs  in  certain  species  of  Quercus,  Cas- 
uarina,  and  the  Ericaceae  the  large  ray  is  in  sharpest  contrast  to 
the  primitive  uniseriate  condition. 

In  the  foregoing  paragraphs  a  general  account  has  been  sup- 
plied of  the  various  anatomical  features  of  dicotyledonous  woods, 
and  an  attempt  has  been  made  to  indicate  the  primitive  condition 
in  connection  with  each  category  of  structures.  It  must  not, 
however,  be  supposed  that  a  primitive  condition  of  organization 
in  regard  to  any  one  of  the  categories  described  in  the  preceding 
pages  necessarily  indicates  for  a  given  group  a  low  position  in  the 
evolutionary  scale.  Taken  altogether,  nevertheless,  they  supply 
extremely  valuable  testimony  from  the  standpoint  of  the  doctrine 
of  descent  and  on  the  whole  the  best  available  in  the  present  state 
of  our  ignorance  regarding  fossil  ancestors  of  the  angiosperms. 


384  THE  ANATOMY  OF  WOODY  PLANTS 

It  will  accordingly  be  of  interest  in  the  present  connection  to 
summarize  the  evidence  in  regard  to  the  primitive  type  of  dicotyle- 
don supplied  by  anatomical  data. 

A  tendency  of  long  standing  is  to  consider  the  amentiferous 
forms  as  representing  a  low  condition  among  the  dicotyledonous 
angiosperms.  In  discussions  in  this  connection  it  is  well  to  dis- 
tinguish the  true  Amentiferae  from  types  which  simulate  them. 
The  Salicales,  for  example,  on  the  grounds  both  of  their  anatomical 
organization  and  of  important  details  of  floral  structure,  cannot 
be  regarded  as  allied  in  any  but  the  remotest  way  with  types  like 
the  alder  and  the  oak.  Further,  on  anatomical  ground  types 
with  more  or  less  well-organized  floral  structure  must  be  admitted 
to  affinity  with  the  Betulaceae,  Fagaceae,  etc.  This  is  true  of 
the  Casuarinaceae  and  Ericaceae.  On  the  basis  of  the  co-ordinate 
occurrence  of  vessels  with  scalariform  perforation,  tracheary 
fibrous  elements,  diffuse  wood  parenchyma,  and  aggregate  rays 
we  must  accord  to  the  genus  Casuarina  a  primitive  position  among 
the  dicotyledons.  This  designation  of  affinity  on  the  basis  of  the 
organization  of  the  wood  is  supported  by  the  fact  that  it  alone 
among  the  dicotyledons  possesses  transfusion  tissue  of  the  flanking 
gymnospermous  type.  A  further  indication  of  its  primitive 
position  is  furnished  by  the  presence  within  the  ovules  of  excep- 
tionally large  amounts  of  sporogenous  tissue  and  also  of  tracheary 
elements.  Finally,  we  have  the  phenomenon  of  chalazogamy, 
the  significance  of  which  is  still  much  disputed.  On  anatomical 
grounds  there  could  scarcely  be  stronger  reasons  for  regarding 
Casuarina  as  the  most  primitive  representative  of  the  dicotyledons. 
The  only  objection  that  has  been  seriously  urged  against  this 
position  is  the  fusion  of  parts  in  the  ovuliferous  floral  structures. 
This  objection,  however,  must  weigh  lightly  in  the  balance  in 
view  of  our  knowledge  of  the  extreme  conservatism  of  anatomical 
structures  in  the  case  of  living  and  extinct  gymnosperms.  It  has, 
for  example,  been  pointed  out  that  the  living  cycads  are  practically 
identical  in  anatomical  organization  with  the  extinct  bennettitean 
forms,  although  their  reproductive  structures  differ  very  widely. 
Fusion  of  floral  parts  does  not  furnish  a  sufficient  argument  for 
a  high  systematic  position,  as  on  that  ground  the  genus  Welwitschia 


THE  WOODY  DICOTYLEDONS  385 

would  be  put  in  a  higher  taxonomic  position  than  Ranunculus, 
because  its  reproductive  structures  present  a  condition  of  cohesion 
not  found  in  the  flower  of  the  buttercup.  The  suggestion  that 
Casuarina  owes  its  anatomical  organization  to  its  xerophytic 
habit  must  be  definitely  rejected  because  of  the  extremely  general- 
ized type  presented  by  the  organization  of  its  woody  structures. 
In  the  single  genus  under  consideration  all  the  types  of  rays  found 
in  dicotyledons,  as  has  been  shown  in  an  earlier  chapter,  are 
presented  in  the  different  species.  Since  all  the  species  are  equally 
xerophytic,  it  is  quite  impossible  to  connect  any  type  of  radial 
parenchymatous  organization  with  the  xerophilous  habit.  Physio- 
logical or  ecological  explanations  of  anatomical  facts  are  always 
to  be  welcomed  when  they  have  any  logical  probability,  but  when 
they  fail  in  this  respect  they  only  obscure  the  evolutionary  situa- 
tion. The  general  anatomical  evidence  in  the  case  of  the  interest- 
ing genus  Casuarina  seems,  in  the  present  state  of  our  knowledge 
at  any  rate,  entirely  to  justify  the  primitive  position  assigned 
to  it  in  the  great  systematic  work  of  Engler  and  Prantl,  in  which 
it  is  placed  at  the  very  base  of  the  dicotyledons. 

If  on  anatomical  and  other  grounds  Casuarina  must  be  regarded 
as  a  primitive  representative  of  the  dicotyledonous  angiosperms, 
it  is  equally  clear  on  the  same  evidence  that  with  it  must  be  joined 
the  Betulaceae  and  Fagaceae  and  in  all  probability  the  Ericaceae. 
The  usual  fusion  of  parts  in  the  flowers  of  the  groups  enumerated 
cannot  apparently,  in  view  of  the  overwhelming  anatomical 
evidence  for  their  primitive  position,  be  regarded  as  a  very  im- 
portant systematic  criterion.  Wind  pollination  and  the  general 
absence  of  herbaceous  forms  further  supply  striking  features  of 
conformity  to  the  conditions  found  in  the  higher  gymnosperms 
from  which  it  is  very  likely  the  dicotyledons  have  taken  their 
origin. 

Another  assemblage  of  forms  which  has  been  pushed  into  the 
foreground,  particularly  in  recent  years,  as  the  primitive  repre- 
sentatives of  the  dicotyledonous  angiosperms  are  the  Ranales. 
Without  discussing  whether  the  families  included  under  this 
general  heading  all  properly  belong  here,  we  may  point  out  that 
on  anatomical  grounds  the  claims  of  that  group  to  affinity  with 


386  THE  ANATOMY  OF  WOODY  PLANTS 

the  Gnetales  and  other  gymnospermous  types  must  be  regarded 
as  somewhat  doubtful.  The  general  anatomical  organization  of 
the  Ranales  presents  some  range  of  variety;  but  they  do  not 
strikingly  exhibit  either  vessels,  fibers,  parenchymatous  distribu- 
tion, or  organization  of  the  rays  such  as  would  be  expected  from 
the  study  of  the  higher  gymnospermous  types  and  the  investigation 
of  primitive  regions  and  organs  in  the  dicotyledons  themselves 
in  primitive  representatives  of  the  angiosperms.  The  only  strong 
argument  which  can  be  advanced  for  the  low  systematic  position 
of  the  Ranales  is  based  on  their  floral  organization.  The  fact 
that  this  has  been  deliberately  disregarded  in  the  Natilrliche 
Pflanzenfamilien  of  Engler  and  Prantl  shows  that  from  the  sys- 
tematic standpoint  it  is  a  criterion  which  cannot  be  considered 
as  of  overwhelming  importance.  With  the  ranalian  origin  of  the 
angiosperms  is  tied  up  the  whole  hypothesis  of  their  derivation 
from  the  Bennettitales.  It  has  been  pointed  out  that,  from  the 
point  of  view  of  anatomy,  there  seems  to  be  little  reason  to  enter- 
tain the  hypothesis  that  the  proangiosperms  were  of  cycadean 
origin,  even  if  the  earlier  discovered  impressions  of  the  reproductive 
remains  of  Mesozoic  cycadean  forms  were  originally  referred  to 
angiospermous  affinities  by  so  distinguished  a  paleobotanist  as 
Saporta.  Anatomically  speaking,  there  seems  accordingly  to  be 
equally  slight  grounds  for  the  derivation  of  the  angiosperms  from 
zoidogamous  gymnosperms  as  from  ranalian  dicotyledons.  The 
whole  question,  however,  must  await  final  and  satisfactory  solu- 
tion until  we  are  actually  acquainted  with  the  earlier  angiospermous 
types  which  will  doubtless  be  sooner  or  later  brought  to  light  from 
the  Jurassic  or  from  even  earlier  epochs  of  the  Mesozoic.  In 
the  meantime  such  knowledge  of  the  general  anatomical  principles 
as  has  been  gained  from  the  comparison  of  extinct  and  living 
gymnosperms  appears  to  point  toward  types  like  Casuarina  and 
its  allies  as  representing,  at  any  rate  anatomically,  the  most 
primitive  conditions. 


CHAPTER  XXVIII 

THE  HERBACEOUS  DICOTYLEDONS 

The  herbaceous  type  in  the  dicotyledons,  although  not  of 
any  definite  systematic  value,  has  a  considerable  significance 
from  the  anatomical  and  evolutionary  standpoints.  It  has  been 
pointed  out  in  earlier  pages  that  discontinuity  of  the  woody  cylinder 
in  siphonostelic  forms  simulating  that  found  in  herbaceous  dicoty- 
ledons is  frequently  present  in  older  groups.  In  such  cases,  how- 
ever, the  situation  has  a  very  different  anatomical  explanation 
and  a  diverse  evolutionary  interpretation  from  that  presented 
by  the  dicotyledonous  angiosperms.  In  many  of  the  lower  groups 
of  plants  belonging  to  both  Lycopsida  and  Pteropsida  we  find  the 
surviving  representatives  distinguished  from  those  characteristic 
of  the  period  of  greatest  luxuriance  by  a  herbaceous,  as  opposed 
to  an  arboreal,  habit.  Such  herbaceous  types  owe  their  origin 
to  degenerative  changes  and  present  a  distinct  contrast  to  the 
situation  exemplified  in  the  herbaceous  dicotyledons,  where  the 
modification  is  the  result  of  differentiation  and  specialization  and 
not  the  consequence  of  mere  degeneracy. 

It  is  necessary  to  illustrate  clearly  the  mode  of  appearance  of 
the  herbaceous  type  in  the  older  groups  before  passing  on  to  the 
consideration  of  the  situation  manifest  in  the  herbaceous  angio- 
sperms. Fig.  2660,  illustrates  the  organization  of  a  siphonostelic 
stem  in  a  lepidodendrid.  Here  the  primary  wood  is  well  developed 
and  constitutes  a  continuous  cylinder.  The  secondary  xylem, 
which  is  the  result  of  cambial  activity  on  the  inside  of  the  primary 
phloem,  forms  a  continuous  layer  interrupted  only  in  the  region 
of  exit  of  a  strand  destined  to  a  branch,  which  causes  a  gap  in  the 
primary  cylinder  that  is  naturally  perpetuated  in  the  earlier 
organization  of  the  secondary  cylinder.  In  b  is  shown  the  condi- 
tion presented  by  a  sigillarian  of  Permian  age.  In  this  instance 
the  siphonostelic  primary  cylinder  has  become  degenerate  and 
consequently  is  only  well  developed  hi  the  regions  facing  departing 

387 


388 


THE  ANATOMY  OF  WOODY  PLANTS 


foliar  traces.  As  a  direct  result  of  this  discontinuity  of  the  primary 
cylinder,  the  secondary  cylinder  becomes  united  only  by  the 
gradual  broadening  of  the  originally  separate  secondary  segments. 
In  c  is  portrayed  the  condition  found  in  the  stem  of  a  third  rep- 
resentative of  the  Lycopsida — namely,  a  calamite.  Here  the 
primary  wood  is  extremely  degenerate,  and  the  secondary  segments 
as  a  consequence  are  very  narrow  and  quite  widely  separated. 

In  the  herbaceous  living 
Equisetum,  the  stem  of 
which  is  depicted  on  an 
earlier  page,  secondary 
growth  has  disappeared, 
being  present  only  as  a 
vestige  in  conservative 
organs  and  regions. 

The  series  indicated 
above  illustrates  the  origin 
of  the  herbaceous  type  by 
degeneracy.  The  Lycop- 
sida have  been  chosen 
because  they  reveal  the 

^  -^ui_ui>-   ^        situation   in   the  clearest 

FIG.  266.— Diagram  showing  the  effect  of  manner  and  with  the 
degeneracy  of  the  primary  wood  on  the  develop-  fewest  complications.  In 
ment  of  the  secondary  cylinder.  Explanation  in  ^  pteropsida  the  topog- 
the  text.  . 

raphy  13  rendered  less  easy 

of  comprehension  by  the  presence  of  the  often  numerous  foliar 
gaps  which  characterize  the  anatomical  organization  of  the 
siphonostelic  central  cylinder  of  that  phylum  of  vascular  plants. 
The  principles  involved  are,  however,  the  same.  It  may  accord- 
ingly be  stated  that  discontinuity  in  the  primary  siphonostelic 
central  cylinder  is  due  either  to  gaps  related  to  the  exits  of  traces 
supplying  the  appendages  or  to  the  local  degeneracy  of  the  cylinder 
itself.  These  interruptions  are  perpetuated  in  the  early  organiza- 
tion of  the  secondary  cylinder.  If  the  secondary  tissues  are  also 
degenerate,  as  in  the  existing  survivors  of  cryptogamic  groups,  a 
pronounced  herbaceous  condition  is  the  result.  This  is,  however, 


THE  HERBACEOUS  DICOTYLEDONS  389 

pre-eminently  the  consequence  of  degeneracy  and  has  no  dynamic 
evolutionary  significance  in  contrast  to  herbaceous  modifications 
presented  by  the  stems  of  dicotyledonous  angiosperms. 

With  the  foregoing  preliminary  statement  in  regard  to  the 
appearance  of  the  herbaceous  type  in  primary  and  secondary 
cylinders  (or  in  the  secondary  cylinder  alone)  in  lower  forms  we 
are  in  a  position  to  attack  the  origin  of  the  stems  of  herbaceous 
texture  in  the  highest  seed  plants.  It  has  been  noted  in  the 
preceding  paragraphs  that  the  degenerate  herb  is  derived  from 
ancestral  forms  characterized  by  woody  stems.  The  same  general 
condition  is  found  in  dynamic  herbaceous  types  among  dicoty- 
ledons; and  it  is  extremely  important  to  keep  this  situation 
clearly  in  view,  as  a  failure  to  do  so  results  in  an  anatomical  fallacy. 
It  cannot  accordingly  be  too  strongly  emphasized  that  a  proper 
understanding  of  herbaceous  axes  in  the  angiosperms  can  be  reached 
only  by  the  comparison  of  nearly  related  stems  of  woody  and 
herbaceous  texture.  Any  other  procedure  leads  to  erroneous 
results. 

In  former  chapters  the  various  types  of  rays  in  the  Gnetales 
and  the  dicotyledons  have  been  discussed.  In  the  present  con- 
nection only  one  of  these  need  seriously  be  considered — the  com- 
pound ray.  The  large  or  compound  type  of  ray  is  characteristic 
of  only  a  few  arboreal  types  of  probably  primitive  antecedents,  but 
occurs  in  a  more  or  less  modified  form  in  many  herbaceous  and 
climbing  stems.  It  will  be  convenient  to  discuss  first  the  condi- 
tions present  in  vines,  as  these  are  usually  more  woody  in  their 
character  and  consequently  serve  as  an  appropriate  transition  to 
axes  of  soft  or  herbaceous  texture.  As  a  preliminary  to  the  con- 
sideration of  the  anatomy  of  the  vine,  it  will  be  well  to  devote 
some  attention  to  the  organization  of  an  exotic  member  of  the 
Vitaceae  from  the  tropics.  Fig.  267  reproduces  the  woody  cylinder 
of  the  shrubby  genus  Leea  from  the  eastern  tropics.  To  the  right 
are  to  be  seen  a  number  of  large  rays  which  clearly  belong  to  the 
category  designated  in  an  earlier  chapter  as  compound.  Above 
and  below,  and  particularly  to  the  left  of  the  illustration,  the  com- 
pound rays  become  much  smaller  in  size.  Those  which  are  of 
greater  dimensions  are  in  relation  to  leaf  traces  passing  out  in  the 


39° 


THE  ANATOMY  OF  WOODY  PLANTS 


region  of  the  node.  Fig.  2680  shows  one  of  these  on  a  higher  scale 
of  magnification.  In  the  large  ray  and  toward  the  bottom  of  the 
figure  lies  the  leaf  trace,  which  is  both  subtended  and  flanked  by 
storage  parenchyma.  The  wood  on  either  side  of  the  large  ray 
shows  the  presence  of  the  primitive  rays,  which,  as  has  been 
pointed  out  on  an  earlier  page,  may  be  considered  in  a  general 
way  as  equivalent  to  the  narrow  rays  of  the  conifers  and  similar 

forms.  In  Fig.  2686  is 
seen  a  vertical  tangen- 
tial section  through  one 
of  the  large  leaf  rays  of 
the  same  genus.  It  is 
evident  that  the  foliar 
trace  runs  in  a  much 
enlarged  ray  and  is  sur- 
rounded on  all  sides  in 
its  horizontal  course  by 
ray  parenchyma.  It  is 
very  clear  from  the  con- 
sideration of  the  genus 
Leea  that  there  are  pres- 
ent large  rays  in  storage 
relation  to  the  leaf 
traces,  precisely  as  in 
the  oak,  Casuarina,  and  similar  forms.  We  may  now  pass  to 
the  situation  in  Vitis,  where  the  conditions  are  not  so  manifest, 
but  are  quite  intelligible  by  comparison  with  the  shrubby  member 
of  the  Vitaceae  just  described.  Fig.  269  shows  the  general  topog- 
raphy of  the  stem  of  the  Concord  grape  in  the  region  of  the  node. 
Seven  leaf  traces  are  to  be  seen  as  darker  masses.  Of  these  only 
one  is  passing  into  the  cortex  and  the  rest  are  still  contained  within 
the  woody  cylinder.  Fig.  2700  illustrates  one  of  the  foliar  seg- 
ments of  the  stem  with  a  higher  degree  of  magnification.  The 
segment  in  question  is  separated  on  either  hand  from  the  rest  of 
the  woody  cylinder  by  broad  radial  bands  of  parenchyma.  In  the 
leaf  trace  itself  may  be  found  radial  dark  stripes,  the  primitive 
rays.  These  are  absent  in  the  segments  of  the  cylinder  which 


FIG.  267. — Transverse  section  of  woody  cylin- 
der of  Leea.    Explanation  in  the  text. 


THE  HERBACEOUS  DICOTYLEDONS  391 

lie  on  either  side  of  the  trace.  The  leaf  trace  thus  illustrates  in 
the  genus  Vitis  the  conservative  character  which  has  been  asserted 
for  it  on  earlier  pages.  The  most  interesting  feature  presented 
by  Fig,  2700  is,  however,  the  fact  that  the  leaf  trace  is  subtended 
externally  by  a  broad  mass  of  parenchyma  which  on  its  flanks 
passes  inwardly  into  the  broad  rays,  separating  the  foliar  segment 
from  its  neighbors  on  either  hand.  The  situation  in  a  general 


FIG.  268. — a,  transverse  section  of  leaf  ray  in  Leea;  b,  vertical  section  of  leaf  ray 
in  Leea. 

way,  in  fact,  duplicates  that  found  in  the  case  of  Leea,  except  that 
primitive  rays  are  confined  to  the  traces  proper  in  Vitis  and  the 
broad  ray  subtending  the  trace  is  very  much  shortened  in  its 
radial  dimension.  Fig.  2706  shows  the  same  trace  in  a  lower 
section  or,  in  other  words,  considerably  below  the  node.  Here 
the  broad  ray  facing  or  confronting  the  leaf  trace  has  disappeared, 
having  been  gradually  replaced  by  typical  woody  tissues,  con- 
sisting of  septate  fibers  and  vessels.  As  a  consequence  of  this 
situation  the  flanking  rays  are  now  separately  continuous  to  the 
outside  of  the  cylinder  and  are  not  united  by  a  broad  tangential 


392 


THE  ANATOMY  OF  WOODY  PLANTS 


mass  of  storage  parenchyma,  as  is  the  case  in  the  region  shown 
in  Fig.  270(1.  Vertical  tangential  sections  make  the  situation 
more  apparent.  Fig.  2710  shows  a  plane  of  section  near  the  surface 
of  the  cylinder.  The  leaf  trace  (seen  as  a  dark  mass)  is  completely 
surrounded  by  storage  parenchyma,  which,  however,  is  less  well 
developed  below  than  on  the  upper  side  of  the  strand.  It  is 
particularly  obvious  in  this  plane  that  the  trace  runs  in  a  mass  of 

storage  parenchyma 
precisely  as  in  Leea, 
described  above;  The 
tangential  sections 
taken  nearer  the  cen- 
ter of  the  cylinder 
present  a  very  differ- 
ent appearance.  In 
this  region  the  trace 
will  still  be  pursuing 
its  vertical  course  in 
the  stem  and  as  a 
consequence  will  be 
flanked  by  storage 

FIG.  269.-Transverse  section  of  the  node  of  the     parenchyma  laterally. 
Concord  grape.    Explanation  in  the  text.  Above   the   trace   lies 

the  parenchyma  of 

the  foliar  gap.  This  statement  may  be  verified  by  reference  to 
Fig.  2716.  The  trace  distinctly  contains  numerous  primitive  rays, 
conspicuously  absent  in  the  segments  on  either  side,  the  segment 
on  the  right  showing  the  presence  of  some  vasicentric  paren- 
chyma, such  as  is  characteristic  of  the  Vitaceae  in  general.  It 
will  now  be  convenient  to  consider  the  topography  of  the  foliar 
trace  in  its  relation  to  the  woody  cylinder  of  the  stem  in  a 
slender  upper  node  of  the  vine.  Fig.  272  reproduces  the  general 
relations  exhibited  by  such  a  thin  axis.  Obviously,  as  the  cyl- 
inder of  the  wood  becomes  more  attenuated,  the  trace  of  the  leaf 
will  increase  in  size  relatively  to  surrounding  parts.  This  situa- 
tion is  clearly  shown  in  the  illustration,  which  represents  a  plane 
of  section  immediately  below  the  node  and  corresponding  in 


THE  HERBACEOUS  DICOTYLEDONS  393 

level  to  that  in  Fig.  2700.  It  is  thus  evident  that,  if  the  woody 
cylinder  is  thin  enough,  the  leaf  trace  will  correspond  to  it  in 
thickness,  and  that  as  a  result  the  subtending  or  confronting 
parenchyma,  so  prominent  a  feature  of  the  topography  of  the  leaf 
trace  in  the  cylinder  of  Leea,  Casuarina,  the  oak,  and  other  woody 
stems  with  compound  rays,  is  plainly  absent.  The  comparison 
of  very  slender  and  thicker  annual  stems  of  the  vine  with  those 


FIG.  270. — a,  transverse  section  of  foliar  ray  in  Vitis  in  the  region  of  the  node, 
b,  transverse  section  of  foliar  ray  below  the  node. 

of  the  shrubby  Leea  makes  it  clear  that  an  extreme  thinning  of 
the  axis  brings  with  it  a  condition  of  organization  in  which  the 
storage  parenchyma  no  longer  surrounds  the  foliar  trace,  but 
merely  flanks  it  on  either  hand.  In  climbing  or  herbaceous  axes, 
in  which  this  slenderness  has  become  a  fixed  feature,  some  com- 
pensation for  the  loss  of  subtending  or  confronting  storage  tissues 
is  provided  by  the  great  lengthening  of  the  flanking  rays,  which 
often  extend,  as  was  pointed  out  by  Strasburger  over  a  quarter 
of  a  century  ago,  through  one  or  more  internodes.  It  will  be 
obvious  from  the  facts  introduced  in  the  present  paragraph  that 


394  THE  ANATOMY  OF  WOODY  PLANTS 

woody  axes  with  the  compound  type  of  foliar  ray,  if  they  become 
sufficiently  slender,  as  in  stems  of  herbaceous  texture,  give  rise 
to  a  condition  in  which  the  storage  parenchyma  is  entirely  lateral 
to  the  leaf  traces. 

The  subject  of  the  organization  of  rays  in  typical  vines  will  be 
made  clearer  by  diagrammatic  representation.  Fig.  273  illustrates 
the  appearance  of  decorticated  stems  of  Leea  and  Vitis  in  three 


FIG.  271. — a,  tangential  section  showing  leaf  trace  surrounded  by  storage  paren- 
chyma; b,  section  of  leaf  trace  and  ray  near  the  pith.     Explanation  in  the  text. 

dimensions.  In  a  is  seen  a  total  view  of  the  nodal  region  of  Leea. 
The  traces  originate  from  the  axis  along  a  crescentic  line  and  are 
shown  in  face  view  on  the  surface  of  the  cylinder  and  in  section 
on  its  cut  end.  Obviously  the  leaf  strands  are  imbedded  in  foliar 
rays  when  viewed  from  either  the  superficial  or  the  transverse 
aspect.  In  addition  to  the  foliar  rays,  which  in  this  case  are 
both  broad  and  shallow,  there  are  other  narrower  rays,  which  are 
usually  of  greater  depth.  There  are  also  uniseriate  rays,  diffused 
throughout  the  structure  of  the  wood  and  presenting  a  rather 
marked  contrast  to  the  two  sorts  of  broad  rays.  In  b  is  shown  a 


THE  HERBACEOUS  DICOTYLEDONS 


395 


magnified  image  of  a  part  of  the  transverse  aspect  of  the  stem, 
making  clearer  the  relations  of  the  leaf  trace  to  the  large  foliar 
ray.  It  is  apparent  that  the  ray  is  similar  to  that  found  in  oaks 
in  cooler  climates  and  that  it  has  a  like  relation  to  the  leaf  trace. 
In  c  is  shown  a  diagram  of  Vitis,  illustrating  the  relations  of  large 
rays  and  foliar  traces.  The  cylinder  in  this  instance  shows  only 
a  single  annual  ring,  and  the  rays  subtending  the  leaf  trace  in 
transverse  section 
are  consequently  less 
deep  in  the  radial 
direction  than  those 
of  Leea.  Further, 
in  the  facial  aspect 
of  the  cylinder  the 
leaf  rays  are  seen  to 
be  much  elongated 
below  and  separated 
by  a  median  process 
of  wood  into  pairs  of 
rays.  This  situation 
is  strikingly  unlike 
that  in  the  shallow 
foliar  rays  of  Leea. 
Still  another  con- 
trast to  the  exotic 
genus  is  offered  by 

the  absence  of  uniseriate  rays  except  in  the  actual  leaf  trace,  an 
interesting  exemplification  of  the  persistence  of  primitive  characters 
in  foliar  organs.  A  more  highly  magnified  view  (d)  of  a  segment 
of  the  transverse  aspect  of  the  cylinder  makes  clear  the  presence  of 
uniseriate  rays  in  the  leaf  trace  and  their  absence  in  the  adjoining 
parts. 

We  may  now  pass  conveniently  to  the  diagrammatic  comparison 
of  the  topographical  relations  presented  by  thicker  and  thinner 
annual  stems  of  Vitis.  In  Fig.  2740  is  shown  a  view  of  a  thicker 
axis  from  the  surface  of  attachment  of  a  leaf  at  the  median  node. 
The  upper  part  of  the  figure  shows  a  transverse  view  of  the  stem 


FIG.  272.— Transverse  section  of  part  of  a  thin  axis 
of  Vitis.    Explanation  in  the  text. 


396 


THE  ANATOMY  OF  WOODY  PLANTS 


just  below  the  next  superior  node.  In  the  latter  the  foliar  traces, 
seven  in  number,  are  clearly  subtended  by  a  greater  or  less  depth 
of  storage  parenchyma,  precisely  as  is  the  case  in  the  oak.  In  b 
the  foliar  trace  is  shown  in  its  radial  and  tangential  aspects.  In 
the  radial  view  the  trace  is  confronted  in  its  upper  region  by 
storage  parenchyma  (black),  while  below  it  is  subtended  by  ordi- 
nary wood.  Above  the  trace  lies  the  storage  tissue  of  the  leaf  gap. 
In  the  tangential  figure  the  trace  appears  as  a  light  spot  in  a  mass 


FIG.  273. — Diagram  of  decorticated  stems  of  Leea  and  Vitis  in  three  dimensions. 
Explanation  in  the  text. 

of  darkly  rendered  storage  parenchyma.  The  latter,  farther  down, 
is  divided  into  two  by  a  median  mass  of  unmodified  wood.  In  c 
is  shown  a  view  of  a  slender  stem.  In  this  instance,  for  variety, 
the  opposite  or  tendril  side  of  the  stem  is  exposed  to  observation. 
On  account  of  the  shallow  diameter  of  the  woody  cylinder  the 
trace  is  of  a  radial  dimension  so  great  that  it  equals  that  of  the 
cylinder  from  which  it  is  derived.  As  a  consequence  there  is  no 
parenchyma  facing  the  leaf  trace,  and  storage  tissue  is  entirely 
flanking  in  its  distribution.  This  is  a  natural  and  necessary 
geometrical  result  of  the  thinning  of  the  cylinder  of  wood  in  the 
more  slender  axes  of  the  vine  type.  In  d  are  shown  the  radial 
and  tangential  relations  of  the  foliar  traces  and  the  adjacent  storage 


THE  HERBACEOUS  DICOTYLEDONS  397 

tissues.  It  is  clear  in  the  radial  view  that  there  is  in  thin  axes 
no  parenchyma  subtending  the  leaf  traces  immediately  below  the 
node.  This  situation  necessitates  the  tangential  aspect  shown 
below,  where  the  leaf  trace  appears  flanked  by  storage  tissues 
on  either  side.  Plainly,  therefore,  by  reason  of  the  slenderness 
of  the  woody  cylinder  and  the  great  lengthening  of  the  rays  related 
to  the  leaf  traces,  which  may  be  regarded  as  in  some  measure  a 


FIG.  274. — Diagram  of  thicker  and  thinner  axes  of  Vitis.    Explanation  in  the 


text. 


compensation  for  their  small  radial  dimension,  axes  of  the  vine 
type  approximate  very  closely  those  found  in  herbs. 

The  discussion  of  typical  herbs  may  now  be  advantageously 
taken  up;  and  it  may  here  be  remarked  that,  although  necessary 
brevity  makes  it  impossible  to  consider  more  than  a  few  instances 
of  the  origin  of  the  herbaceous  stem,  a  wide  investigation  of  the 
situation  in  many  groups  of  herbaceous  dicotyledons  has  made 
it  clear  that  the  same  general  conditions  are  present  in  every 
instance.  Fig.  275  reproduces  part  of  a  transverse  section  through 
the  upper  slender  region  of  the  stem  of  the  common  stinging 


THE  ANATOMY  OF  WOODY  PLANTS 


nettle,  chosen  on  account  of  its  chalazogamous  affinities  for  com- 
parison with  Casuarina,  Betula,  Quercus,  etc.  In  the  illustration 
a  leaf  trace  with  its  flanking  broad  rays  is  shown,  as  well  as  the 
adjacent  segments  of  the  cylinder.  No  primitive  rays  are  to  be 
seen  in  the  trace,  which  is  composed  of  vessels  and  wood  fibers. 
In  order  properly  to  interpret  the  slender  axis  of  the  nettle,  the 
stouter  and  more  woody  regions  of  the  stem  should  be  examined. 

Fig.  2760  repro- 
duces the  appear- 
ance of  a  part  of  a 
section  through  the 
thick  stem  of  the 
species  under  con- 
sideration, taken 
immediately  below 
the  node.  The 
trace  appears  as  a 
highly  vascularized 
radially  directed 
mass  on  the  mar- 
gin of  the  pith. 
On  either  side  of  it 
are  radial  bands  of 
parenchyma,  which 
correspond  to 
those  shown  lateral 

to  the  leaf  traces  in  Fig.  275.  In  addition  to  flanking  storage 
tissue  the  woody  region  of  the  stem  of  Urtica  shows  a  very  mas- 
sive band  of  radial  storage  tissue  confronting  the  leaf  trace,  com- 
parable to  the  similar  structures  found  in  the  case  of  the  oak 
and  Leea.  The  banded  appearance  of  the  broad  foliar  ray  in 
this  instance  is  due  to  the  presence  of  alternating  stripes  of  true 
parenchyma  and  substitute  fibers.  A  region  of  the  axis  farther 
below  the  node  may  now  be  considered.  Fig.  2766  shows  a  part 
of  a  transverse  section  of  the  stem  some  distance  below  the 
node.  The  mass  of  confronting  storage  tissue  in  the  region  of 
the  node  at  the  lower  level  has  become  transformed  centrally 


FIG.  275. — Transverse  section  through  part  of  a  slen- 
der stem  of  the  nettle.    Explanation  in  the  text. 


THE  HERBACEOUS  DICOTYLEDONS  399 

into  typical  wood,  precisely  as  has  been  shown  above  to  occur 
in  the  vine.  A  careless  study  of  the  facts  and,  above  all,  a  neglect 
to  examine  the  conditions  present  in  the  region  of  the  node,  might 
lead  to  the  conclusion  that  the  leaf  trace  in  the  more  primitive 
and  lower  woody  portion  of  the  stem  in  Urtica  is  flanked  but  not 
confronted  by  storage  parenchyma. 

Although  the  Urticaceae  have  been  chosen  to  illustrate  the 
herbaceous  condition  in  a  comparatively  low  order  of  dicotyledons, 


FIG.  276. — a,  part  of  the  thick  stem  of  the  nettle  immediately  below  the  node; 
b,  section  of  the  same  some  distance  below  the  node.    Explanation  in  the  text. 

the  Ranunculaceae,  as  represented  by  the  woody  and  herbaceous 
species  of  Clematis  or  the  more  slender  and  thicker  region  of  the 
same  woody  species,  yield  like  results.  A  group  in  an  approxi- 
mately intermediate  position  systematically  is  the  Rosaceae. 
Herbaceous  representatives  when  compared  with  woody  forms 
of  allied  organization  yield  similar  conclusions.  For  example, 
in  Rubus  we  find  well-marked  compound  rays,  which  both  flank 
and  subtend  the  foliar  traces.  In  many  herbaceous  types  of  the 
genus  Potentilla  broad  rays  like  those  of  the  oak  are  found  in  the 


400 


THE  ANATOMY  OF  WOODY  PLANTS 


lower  regions  of  the  aerial  axis,  while  in  the  upper  parts  the  con- 
fronting parenchyma  is  eliminated  from  the  cylinder  as  a  result 
of  its  progressive  reduction  in  thickness,  precisely  as  in  Vitis 
and  Urtica. 

The  Compositae,  so  generally  conceded  a  very  high  systematic 
position  among  the  dicotyledons,  exemplify  exactly  the  same 
principles  as  have  been  noted  above.  Fig.  277  represents  a  low 

region  of  a  somewhat 
woody  axis  of  Helianthus 
hirsutus.  The  leaves, 
opposite  as  in  Urtica, 
have  three  traces  each. 
The  foliar  strands  corre- 
spond to  depressed  seg- 
ments of  the  stem,  a 
situation  which,  as.  has 
been  noted  in  an  earlier 
chapter,  is  commonly 
found  in  the  vicinity  of 
large  storage  rays  and 
which  results  from  the 
greater  amount  of  vege- 
table substance  present 

in  such  segments  and  a  consequent  retarded  rate  of  growth  as  com- 
pared with  the  more  woody  regions  of  the  cylinder.  In  H.  tuberosus 
the  fluting  of  the  lower  part  of  the  stem  in  the  region  of  the  storage 
rays  related  to  the  leaves  becomes  extremely  marked.  As  there  are 
six  leaf  traces  at  a  given  node,  it  follows  that  there  are  six  cor- 
responding furrows  in  the  internode  below.  Further,  since  the 
traces  of  successive  internodes  alternate,  the  furrows  of  necessity 
show  a  similar  alternation.  The  narrow  bundles  of  greater  diam- 
eter in  the  figure  represent  the  foliar  traces  of  the  next  higher 
node,  and  the  masses  of  sclerenchyma  subtending  the  median 
region  of  the  phloem  of  the  broadest  bundles  correspond  to  the 
now  fused  traces  of  a  still  higher, node  of  the  axis.  Fig.  278 
reproduces  a  high  and  herbaceous  region  of  the  aerial  axis  of 
H.  hirsutus.  The  figure  is  equivalent  in  nodal  relations  to  the  fore- 


FIG.  277. — Thick  stem  of  Helianthus  hirsutus 


THE  HERBACEOUS  DICOTYLEDONS 


401 


FIG.  278.— Slender  upper  region  of  the  stem  of 
//.  hirsutus. 


going,  but  the  topography  is  strikingly  different.  Here  the  foliar 
traces,  instead  of  merely  standing  flush  with  the  surface  of  the 
cylinder,  as  in  the 
upper  region  of  the 
more  woody  herba- 
ceous types,  are  actu- 
ally outstanding,  or 
salient.  As  a  conse- 
quence of  this  situa- 
tion the  leaf  traces  no 
longer  correspond  to 
depressions  of  the 
stem,  but  actually 
underlie  ridges  on  its 
surface.  This  condi- 
tion is,  in  fact,  highly 
characteristic  of  ex- 
treme herbs,  in  which 
the  leaf  trace  has  become  the  dominant  factor  in  the  organization 

of  the  cylinder  of  the 
axis.  This  ana- 
tomical situation 
corresponds  to  a 
very  high  degree  of 
assimilative  effi- 
ciency on  the  part  of 
foliar  organs,  result- 
ing  in  a  large 
amount  of  food 
storage  or  seed  pro- 
duction, as  the  case 
may  be. 

In  Fig.  279  is 
shown  the  trace  of 
the  herbaceous  re- 

FIG.   27o.-Section  showing  the  leaf  trace  in  the     gion  more  highly 
herbaceous  region  of  Helianthus.  m  a  g  n  1  fi  e  d  .     The 


4O2 


THE  ANATOMY  OF  WOODY  PLANTS 


foliar  strand  projects  beyond  the  surface  of  the  cylinder  and  is 
merely  flanked  and  not  subtended  by  storage  parenchyma,  as  in 
similar  regions  in  other  herbaceous  stems  of  the  most  varied 
affinities.  In  contrast,  in  Fig.  280,  which  is  from  a  lower  and 
more  woody  node,  a  large  amount  of  confronting  storage  tissue  is 
seen,  as  well  as  that  present  on  the  flanks  of  the  trace. 

The  topographi- 
cal conditions  in  the 
stem  of  Helianthus, 
as  representing  a 
high  and  typical 
herbaceous  condi- 
tion, may  now 
advantageously  be 
depicted  in  stereo- 
diagram.  Fig.  28 la 
reproduces  the 
lower  portion  of 
the  stem  of  Helian- 
thus.  The  scar  of 
the  leaf  of  one  node 
faces  the  observer, 
while  the  region 
just  below  the  next 
higher  node  is  shown  in  transverse  section.  The  six  traces  of 
the  two  opposite  leaves  are  clearly  seen,  and  it  can  be  readily 
observed  that  they  are  both  flanked  and  faced  by  storage  tissue 
(black).  The  very  narrow  deep  bundles  in  the  cross-sec  don 
represent  leaf  traces  of  the  next  higher  node,  while  the  light  masses 
on  the  periphery  of  the  broad  remaining  strands  indicate  the 
position  of  the  fused  foliar  traces  of  a  still  higher  node.  In  b 
is  shown  (above)  a  radial  and  (below)  a  tangential  view  of  the 
topographical  relations  of  the  leaf  traces  in  these  planes.  In 
the  upper  figure  of  b  there  is  obviously  much  storage  tissue  con- 
fronting the  trace.  In  the  lower  item  the  trace  in  tangential 
section  is  seen  entirely  surrounded  by  storage  tissue.  Fig.  28 ic 
is  a  picture  of  the  solid  relations  of  a  higher  part  of  the  stem  in 


FIG.  280. — Section  of  trace  in  thick  woody  region  of 
Helianthus. 


THE  HERBACEOUS  DICOTYLEDONS 


403 


the  sunflower.  In  this  region  the  foliar  traces,  instead  of  being 
depressed  below  the  level  of  the  woody  cylinder,  are  outstanding, 
a  condition  very  commonly  found  in  extreme  herbs,  which  have 
largely  lost  their  woody  texture.  In  the  case  of  the  vine  the  radial 
diameter  of  the  trace  in  the  slender  region  of  the  stem  merely 
equals  that  of  the  cylinder,  while  in  strongly  herbaceous  types 
the  leaf  trace  is  outstanding  or  salient.  In  the  high  part  of  the 
sunflower  stem  the  trace  is  only  flanked  by  storage  tissue  in  contrast 


FIG.  281. — a,  diagram  of  lower  region  of  stem  of  Helianthus  hirsutus;  b,  radial 
and  tangential  view  of  the  topographical  relations  of  leaf  traces  in  a;  c,  diagram  of  the 
upper  region  of  the  stem  in  the  sunflower;  d,  topography  of  the  trace  in  radial  and 
tangential  aspects.  Full  explanation  in  the  text. 

to  the  both  flanking  and  facing  topography  of  the  storage  paren- 
chyma in  the  lower  and  more  woody  part  of  the  axis.  It  will  be 
noted  that  in  the  high  portion  of  the  axis  the  foliar  traces  cor- 
respond to  elevations  or  ridges  on  the  surface  of  the  stem,  while 
in  the  regions  lower  down  they  subtend  longitudinal  furrows.  In 
Fig.  28 1 d  the  topography  of  the  foliar  trace  is  represented  in 
radial  and  tangential  aspect.  In  the  upper  figure  the  absence 
of  subtending  parenchyma  can  be  clearly  discerned,  while  in  the 
lower  one  the  flanking  disposition  of  the  storage  tissues  is  apparent. 


404 


THE  ANATOMY  OF  WOODY  PLANTS 


It  will  be  advantageous  at  this  stage  to  compare  the  woody 
region  of  the  herbaceous  stem  of  Helianthus  with  the  axis  of  Ca- 
suarina.  Fig.  2820  represents  diagrammatically  a  Casuarina  with 
compound  foliar  rays.  The  best  developed  of  the  rays  are  di- 
rectly related  to  leaf  traces,  while  the  intervening  less  pronounced 
ones  belong  to  the  foliar  strands  of  another  node.  In  order  that 
the  vertical  relations  of  the  foliar  bundles  to  storage  rays  may 


FIG.  282. — Stems  of  Casuarina  and  Helianthus.     Explanation  in  the  text 

be  seen,  the  bark  of  the  stem  is  removed  on  the  side  facing  the 
reader.  The  distant  aspect  still  retains  its  bark  as  well  as  its 
leaf  bases,  which  are  represented  as  pyramidal  elevations  on  the 
surface.  In  Fig.  2826  is  seen  a  corresponding  diagram  of  the 
lower  region  of  a  stem  in  Helianthus.  The  topographical  condi- 
tions are  virtually  the  same,  but  for  the  fact  that  there  are  three 
leaf  traces  to  each  leaf  in  the  sunflower,  instead  of  a  single  one,  as 
in  Casuarina. 

In  the  oak  the  longitudinal  depressions  of  the  woody  cylinder 
do  not  correspond  to  rays  belonging  to  a  single  leaf  trace,  but 


THE  HERBACEOUS  DICOTYLEDONS 


405 


to  pairs  of  these  related  to  the  foliar  strands  of  diverse  leaves  at 
different  nodes.  This  phenomenon  is  rather  rare  in  the  herbaceous 
type,  but  is  occasionally  found  in  the  genera  Aster  and  Solidago, 
among  the  Compositae.  Fig.  283  represents  an  axis  of  this  type 
in  Aster  multiflorus.  There  are  five  depressed  segments  correspond- 
ing to  five  pairs  of  foliar  traces,  to  be  seen  in  the  woody  cylinder. 
Fig.  2840  is  a  diagram  of  the  topography  of  the  axis  in  the  oak. 
The  bark  is  represented  as  removed  on  the  side  toward  the  reader 
and  as  still  present  on  the 
opposite  side.  To  the 
right  on  the  distant 
aspect  of  the  stem  is 
shown  a  leaf  base.  Into 
this  run  three  traces. 
Since  the  median  one 
passes  out  at  a  different 
level  from  the  other  two 
and  exercises  no  impor- 
tant influence  on  the 
topography  of  the  stem, 
it  is  represented  by  a 
dotted  outline.  It  is  to 
be  noted  that  the  lateral 
traces  take  their  origin 
from  the  foliar  rays  which  are  most  remote  from  the  median  trace. 
On  the  left  of  the  posterior  aspect  of  the  axis  is  shown  by  a  dotted 
outline  a  leaf  base  of  a  succeeding  node.  The  traces  of  this  leaf 
are  plotted  in  by  broken  outlines,  so  as  to  indicate  their  relation 
to  the  foliar  rays  of  the  stem.  It  is  clear  from  the  illustration 
that  the  depressed  segment  lying  uppermost  in  the  diagram  is 
flanked  by  two  foliar  rays  which  belong  to  the  lateral  traces  of 
two  successive  leaves.  The  depressed  segment  of  the  woody 
cylinder  in  this  instance  owes  its  position  to  the  retarding  effect 
on  growth  of  the  two  closely  approximated  foliar  rays  which  bound 
it  on  either  side.  In  the  case  of  stems  in  which  the  large  rays  are 
equidistant  there  are  no  depressions,  showing  clearly  that  the 
depressed  segment  is  the  result  of  growth  mechanics.  This  is 


FIG.  283.— Transverse  section  of  the  stem  of 
Aster  multiflorus. 


4o6 


THE  ANATOMY  OF  WOODY  PLANTS 


clearly  exemplified  in  certain  species  of  Clematis.  In  the  numerous 
species  in  which  the  large  rays  are  approximated  in  pairs  there  are 
depressed  segments  in  the  stem;  but  in  Clematis  paniculata,  in 
which  the  large  compound  rays  are  equidistant,  the  segments  of 
the  woody  cylinder  are  all  on  a  level  and  there  are  no  depressed 
regions.  It  is  not  possible  to  follow  this  subject  further  in  the 
present  connection,  but  it  is  enough  to  point  out  that  the  facts 


r 


FIG.  284. — Diagram  of  the  topography  of  the  cylinder  in  Quercus  and  Aster. 
Explanation  in  the  text. 

entirely  invalidate  the  interpretation  of  depressed  segments  ad- 
vanced by  Sanio,  Sachs,  and  De  Bary  and  now  classic  in  botanical 
textbooks.  The  view  of  these  authors  is  that  the  depressed  seg- 
ments, where  they  occur,  owe  their  topography  to  a  later  forma- 
tion than  the  rest  of  the  cylinder.  This  is  distinctly  not  the  case. 
The  classic  view  just  mentioned  further  involves  the  equally 
erroneous  hypothesis  that  the  continuous  cylinder  of  woody  forms 
has  been  derived  from  the  discontinuous  one  of  herbaceous  types, 
a  conclusion  which,  on  the  evidence  furnished  in  the  present  and 
previous  chapters,  is  exactly  opposite  to  the  true  situation.  In 


THE  HERBACEOUS  DICOTYLEDONS  407 

Fig.  2846  is  shown  a  diagrammatic  view  of  the  stem  in  the  genus 
Aster.  Obviously  exactly  the  same  general  conditions  are  found 
as  in  the  axis  of  the  oak. 

The  result  of  all  the  evidence  considered  in  the  previous  para- 
graphs of  the  present  chapter  is  to  show  that  the  herbaceous  type 
of  stem  is  distinctly  derived  from  the  woody  one  in  the  dicotyledons. 
Its  earlier  expression  is  in  the  rather  thick  stem  of  herbaceous 
texture,  in  which  the  storage  parenchyma  especially  related  to 
the  foliar  traces  both  subtends  and  flanks  the  fibrovascular  bundles 
of  the  leaf  in  that  part  of  their  course  which  lies  within  the  cylinder 
of  the  stem.  With  subsequent  thinning  of  the  cylinder,  clearly 
connected  with  greater  efficiency  in  the  production  of  seed  and 
coupled  with  a  more  strictly  annual  duration,  the  subtending 
parenchyma  is  wiped  out,  and  only  the  flanking  storage  tissues 
continue  to  exist.  Obviously  a  very  high  potency  in  the  produc- 
tion of  seed,  correlated  with  high  assimilative  power,  ultimately 
makes  for  a  type  of  annual  stem  in  which  storage  is  effected  mainly 
in  the  seeds  and  no  longer  in  the  axis  itself.  Under  these  circum- 
stances we  have  the  herbaceous  type  reaching  its  extreme  sim- 
plification. The  situation  just  pictured  involves  a  final  marked 
reduction  in  the  organization  of  the  fibrovascular  strands  which 
is  expressed  in  its  completest  form  in  the  bundle  system  of  the 
axes  of  the  monocotyledons.  This  group,  however,  will  be  con- 
sidered in  the  following  chapter. 

The  result  of  the  appearance  of  the  herbaceous  type  in  the 
angiosperms  has  been  momentous  for  the  development  of  higher 
organisms.  It  is  a  fact  of  very  obvious  significance  that  the 
highest  vertebrates  and  the  highest  seed  plants  have  had  a  nearly 
contemporaneous  existence.  The  warm-blooded  mammal  is  in 
reality  rendered  possible  by  the  appearance  of  the  herbaceous 
type  in  the  angiosperms,  which  directly  or  indirectly  supply  the 
most  important  part  of  the  food  of  all  the  higher  animals.  It 
should  further  be  emphasized  in  the  present  connection  that  the 
dicotyledonous  herbs  are  very  different  indeed  in  their  mode  of 
origin  from  those  exemplified  by  lower  types  and  in  particular 
by  the  vascular  cryptogams.  Here  the  herbaceous  condition  is 
the  result  of  mere  degeneracy  in  the  tissues  of  the  xylem.  In 


408  THE  ANATOMY  OF  WOODY  PLANTS 

the  herbaceous  dicotyledons,  on  the  contrary,  we  find  the  origin 
of  herbaceous  texture  closely  associated  with  special  modifications 
in  the  organization  of  the  secondary  cylinder,  and  this  involves 
a  high  degree  of  specialization  on  the  part  of  the  plants  in  which 
it  is  manifested.  The  local  transformation  of  the  tissues  of  the 
woody  cylinder,  in  relation  to  improved  storage  in  proximity  to 
the  leaf  trace,  had  its  early  expression  in  the  phenomenon  of 
aggregation  of  rays  in  proximity  to  this  in  its  course  through  the 
woody  cylinder.  The  phenomenon  of  aggregation  is  succeeded 
in  turn  by  that  of  compounding,  the  direct  consequence  of  the 
facility  with  which  the  longitudinal  elements  of  the  xylem  become 
transformed  into  storage  elements  in  the  case  of  the  dicotyledons. 
The  final  result  achieved  in  the  compound  ray  is  the  wedding  of 
longitudinal  and  radial  parenchyma  in  the  complex,  which  becomes 
of  such  marked  significance  in  the  herbaceous  dicotyledons. 


CHAPTER  XXIX 
THE  MONOCOTYLEDONS 

This  group  of  plants,  although  only  about  one-fifth  as  numer- 
ous as  the  dicotyledons,  is  nevertheless  of  great  importance  on 
account  of  its  significance  in  supplying  extremely  valuable  food 
plants  and  also  by  reason  of  its  remarkable  anatomical  organiza- 
tion, which  has  been  the  'cause  of  much  speculation  and  dispute. 
In  this  large  group  of  the  angiosperms  we  have  exemplified  a 
practically  complete  absence  of  secondary  growth  coupled  with 
a  complicated  arrangement  of  the  numerous  though  slender 
fibro vascular  strands.  The  absence  of  secondary  growth  has  led 
to  the  association  of  the  monocotyledonous  angiosperms  with 
the  ferns  and  their  allies.  This  view  of  their  affinities  has  been 
reinforced  at  various  times  by  the  discovery  of  leaves  and  even 
inflorescences  in  Paleozoic  strata  which  have  been  referred  to 
monocotyledonous  affinities.  The  foliar  and  reproductive  parts 
from  Paleozoic  deposits  have,  however,  in  more  recent  times 
been  clearly  recognized  as  the  parts  either  of  cryptogamous  or 
of  gymnospermous  types  not  related  even  remotely  to  the  angio- 
sperms. There  is  accordingly  no  reason  based  either  on  the 
possession  of  cryptogamic  characters  or  on  very  ancient  occurrence 
as  fossils  which  justifies  the  view  that  the  monocotyledons  are 
the  more  primitive  group  of  the  angiosperms.  The  evidence, 
in  fact,  when  considered  in  the  light  of  the  general  principles  of 
comparative  anatomy,  points  in  quite  the  opposite  direction  and 
seems  to  indicate  that  the  group  under  consideration  is  derived 
from  herbaceous  representatives  of  the  dicotyledons. 

A  characteristic  feature  of  organization  of  the  axis  in  the 
monocotyledons  is  the  scattered  distribution  of  the  fibrovascular 
bundles.  These,  instead  of  being  arranged  in  the  circular  fashion 
which  usually  distinguishes  the  structure  of  the  herbaceous  stem 
in  the  dicotyledons,  are  disposed  through  the  transverse  section 
of  the  organ.  An  examination  of  the  seedlings  and  reproductive 

409 


410  THE  ANATOMY  OF  WOODY  PLANTS 

axes  in  the  group  supplies  convincing  evidence  that  the  peculiar 
arrangement  of  the  fibrovascular  bundles  in  the  monocotyledons 
is  not  a  primitive  one.  The  original  manner  of  distribution  of 
the  conducting  strands  of  the  stem  was  in  all  probability  that 
found  in  the  dicotyledons,  characterized  by  a  prevailing  circular 
arrangement  of  the  bundles. 

The  root  in  monocotyledons  has  the  usual  radial  organization, 
and  is  distinguished  from  that  of  the  mass  of  dicotyledons  by  the 
absence  of  secondary  growth.  Another  feature  which  is  often 
present  in  monocotyledonous  roots  is  the  origin  of  lateral  roots, 
not  opposite  the  groups  of  protoxylem,  as  is  the  general  situation 
in  the  roots  of  the  remaining  vascular  plants,  but  in  the  interval 
between  two  protoxylem  clusters.  This  peculiarity  has  gained 
for  such  roots  the  not  very  appropriate  designation  of  "double 
roots."  Another  feature  which  has  been  described  in  the  roots 
of  the  group  under  consideration  is  found  in  the  abnormal  order 
of  development  of  the  elements  of  the  xylem.  In  vascular  plants 
in  general  the  protoxylem  occupying  the  outside  of  the  xylem 
star  of  the  root  is  differentiated  first  and  the  successively  more 
central  elements  in  later  order.  In  a  number  of  monocotyledonous 
roots  a  remarkable  exception  to  this  well-nigh  universal  seriation 
of  development  has  been  observed,  for  the  more  central  elements 
belonging  to  the  metaxylem  are  differentiated  first  and  the  tra- 
cheids  of  the  protoxylem  are  the  last  to  manifest  the  sculptural 
features  of  maturity. 

The  leaf  in  the  monocotyledons  is  characterized  in  general  by 
the  closed  disposition  of  the  nerves  or  fibrovascular  strands.  These 
usually  come  together  at  the  tip  of  the  foliar  organ  and  sometimes 
in  this  region  are  in  relation  to  rifts  or  pores  in  the  epidermis, 
which  allow  fluid  water  to  escape  in  the  form  of  drops  of  dew.  In 
certain  tropical  plants  with  large  leaves — such,  for  example,  as 
the  Agave — the  loss  of  water  during  the  night  through  the  rifts 
in  the  tips  of  the  leaves  where  the  foliar  bundles  converge  is  so 
great  that  a  constant  dripping  is  heard,  often  so  pronounced  as 
to  disturb  slumber.  As  a  consequence  of  the  closed  disposition 
of  the  fibrovascular  bundles  in  monocotyledonous  leaves,  only 
the  longitudinal  veins  are,  as  a  rule,  well  developed,  and  the 


THE  MONOCOTYLEDONS 


411 


lateral  ones  are  weak  and  degenerate.  The  fibrovascular  bundles 
of  the  leaf  in  monocotyledons  are  ordinarily  very  numerous  and 
consequently  enter  the  stem  in  large  numbers  at  the  nodes.  The 
large  number  of  foliar  bundles  passing  from  the  base  of  the  mono- 
cotyledonous  leaf  into  the  stem  is  correlated  with  a  high  degree 
of  assimilative  efficiency  which  finds  expression  in  a  proportion 
of  seed  production  which  has  scarcely  ever  been  reached  in  her- 
baceous dicotyledons.  In  many  of  the  cereals,  for  example,  the 
relative  weight  of  the  seed  to 
that  of  the  whole  plant  very 
frequently  reaches  over  30  per 
cent.  The  high  efficiency  of  the 
group,  both  from  the  standpoint 
of  production  of  assimilates  and 
from  that  of  the  formation  of 
seeds,  naturally  puts  it  in  a 
unique  position  in  supplying 
important  food  plants. 

In  many  cases,  particularly 
in  the  grasses  and  sedges,  cam- 
bial  activity,  absent  in  the  stem 
and  root,  is  often  retained  in 
the  basal  or  sheath  region  of  the 
leaf  or  sometimes  in  relation  to  the  node  in  the  stem.  The  capacity 
which  grasses  manifest  for  erecting  their  stems  after  "lodging"  is  to 
some  extent  the  result  of  the  presence  of  a  persistent  cambium  in 
the  nodal  region,  either  in  the  base  of  the  leaf  or  in  relation  to  the 
stem  itself.  Fig.  285  illustrates  such  cambial  activity  in  the  case 
of  Avena  barbata.  It  is  permissible  to  view  this  cambial  activity 
as  a  persistence  of  an  ancestral  character,  particularly  as  it  is  often 
found  to  be  present  in  monocotyledonous  seedlings  hi  the  lower 
region  of  the  epicotyl  or  primitive  stem. 

The  organization  of  the  closed  fibrovascular  bundles  of  the 
monocotyledons  is  in  many  cases  collateral  and,  as  the  descriptive 
term  implies,  exhibits  no  indication  of  cambial  activity.  The 
collateral  type  of  fibrovascular  strand  is  characteristic  of  the  leaf, 
since  that  organ,  here  as  elsewhere,  is  conservative  in  its  structure. 


FIG.  285.— Transverse  section  of  a 
bundle  of  Avena  barbata,  showing  cambial 
activity  (after  Chrysler). 


412  THE  ANATOMY  OF  WOODY  PLANTS 

In  the  stem,  however,  particularly  in  the  subterranean  axis,  the 
collateral  type  gives  place  to  a  concentric  condition  in  which 
phloem  is  completely  surrounded  by  xylem.  This  modification 
is  known  as  amphivasal,  to  distinguish  it  from  the  amphicribral 
concentric  strands  of  the  Filicales  and  certain  lower  gymnosperms. 
The  concentric  strands  of  the  monocotyledons  present  themselves 
in  a  very  interesting  fashion  in  the  grasses  and  sedges.  Here,  in 
the  reproductive  axes,  amphivasal  bundles  are  very  numerous 
in  the  nodal  regions,  where  the  entering  of  many  foliar  traces 
produces  a  marked  degree  of  crowding  and  disturbance.  The 
amphivasal  bundle,  in  fact,  seems  to  have  originated  as  a  con- 
sequence of  the  multiplication  of  foliar  traces  in  the  nodal 
regions  of  monocotyledonous  stems.  This  hypothesis  of  the  ori- 
gin of  the  amphivasal  strand  is  justified  by  a  consideration  of 
parallel  conditions  exemplified  in  the  organization  of  the  axis 
in  certain  dicotyledons.  In  many  instances  where  the  foliar 
traces  are  numerous  in  the  nodal  region  of  herbaceous  dicotyledons, 
these  become  amphivasal  in  their  structure.  This  organization, 
for  example,  is  frequently  seen  in  the  Araliaceae  and  Umbelliferae. 
In  the  annual  stems  of  the  mass  of  monocotyledons,  whether 
leafy  or  scapose,  amphivasal  bundles  are  ordinarily  absent.  When 
the  anatomical  structure  of  the  perennial  subterranean  axis  which 
is  often  found  in  the  monocotyledons  is  examined,  it  very  generally 
presents  amphivasal  bundles  in  great  abundance  and  not  by  any 
means  confined  to  the  nodal  regions,  even  when — as  less  rarely 
happens — the  nodes  are  not  closely  approximated.  The  common 
occurrence  of  amphivasal  strands  in  the  rhizomes  of  monocoty- 
ledonous stems  is  no  doubt  primarily  related  to  the  crowded  and 
tufted  character  of  the  leaves  which  results  in  the  strong  approxi- 
mation of  the"  nodes  with  the  consequent  multiplication  of  amphi- 
vasal strands. 

It  is  highly  probable  that  the  glumaceous  representatives  of 
the  monocotyledons  represent  somewhat  primitive  conditions  in 
the  stock,  for  here  both  reproductive  and  anatomical  data  seem 
to  harmonize  in  indicating  a  low  systematic  position  for  both 
grasses  and  sedges.  It  is,  moreover,  probable  that  the  Juncaceae, 
which  in  anatomical  organization  agree  very  closely  with  the 


THE  MONOCOTYLEDONS  413 

glumaceous  monocotyledons,  are  nearly  related  to  these  forms. 
A  striking  contrast  is  presented  in  the  anatomy  of  the  true  palms 
and  the  Scitamineae.  In  these  groups  amphivasal  strands  seem 
to  be  entirely  lacking.  In  the  Principes,  or  true  palms,  we  find  a 
marked  difference  in  anatomical  structure  from  the  western 
tropical  Cyclanthaceae,  which  have  on  floral  grounds  often  been 
considered  to  form  a  systematic  link  between  the  palmlike  mono- 
cotyledons and  the  aroids.  The  anatomical  structure  of  Carlu- 
dovica  and  allied  Cyclanthaceae  is  rather  that  of  the  aroids  than 
of  the  true  palms,  since  amphivasal  strands  are  conspicuously 
present.  Our  knowledge  of  the  development  and  comparative 
anatomy  of  the  groups  which  appear  to  lack  amphivasal  bundles 
is  still  too  meager  to  warrant  any  hypothetical  conclusions  as 
to  the  phylogenetic  significance  of  the  apparent  lack  of  amphivasal 
fibrovascular  strands  in  the  conspicuously  large-leaved  forms, 
which  are  united  systematically  under  the  headings  of  Principes 
and  Scitamineae.  It  may  well  be  that  the  two  large  groups 
above  indicated,  by  the  anatomical  peculiarities  revealed  as  a 
result  of  an  examination  which  is  as  yet  only  preliminary,  occupy 
a  position  high  among  the  monocotyledonous  orders. 

It  will  be  convenient  to  elucidate  in  a  general  way  by  means 
of  diagrams  the  main  anatomical  conditions  presented  by  the 
monocotyledons.  Fig.  2860  illustrates  the  distribution  of  amphi- 
vasal regions  in  the  stem  of  a  sedge.  In  the  reproductive  axis 
the  amphivasal  segments  are  remote  and  are  clearly  in  the  nodal 
regions.  In  proximity  to  the  substratum  the  nodes  become  more 
approximated,  and  in  the  subterranean  axis  they  are  frequently 
so  closely  disposed  that  amphivasal  organization  is  often  contin- 
uous. This  condition  is  well  illustrated  by  the  sedges  and  rushes 
and  less  distinctly  by  the  grasses,  since  the  last  have  well-spaced 
nodes.  The  next  diagram  (6)  shows  the  situation  commonest 
among  the  monocotyledons.  In  this  type  of  anatomical  organ- 
izations the  amphivasal  structures  are  found  exclusively  in  the 
perennial  subterranean  axis  and  do  not  appear  in  the  annual 
stem.  In  the  last  type  (c) ,  which  portrays  the  conditions  apparently 
characteristic  of  the  true  palms  and  the  Scitamineae  (bananas, 
cannas,  ginger,  etc.),  amphivasal  regions  are  absent  both  in  the 


414  THE  ANATOMY  OF  WOODY  PLANTS 

subterranean  and  in  the  aerial  stem.  Only  continued  investiga- 
tion will  show  the  real  significance  of  the  anatomical  situation 
exemplified  in  these  cohorts  or  orders. 


FIG.  286. — Diagrams  to  illustrate  the  distribution  of  amphivasal  bundles  in  the 
monocotyledons. 

The  general  anatomical  configuration  of  the  monocotyledons 
in  the  present  condition  of  our  knowledge  warrants  the  conclusion 
that  this  important  class  or  division  of  the  angiosperms  formerly 
possessed  bundles  arranged  in  a  circular  fashion  and  characterized 
by  secondary  growth.  This  condition  is  indicated  by  the  study 
of  conservative  organs  and  regions  and  seems  to  justify  the  inference 


THE  MONOCOTYLEDONS  415 

that  the  monocotyledonous  angiosperms  have  been  derived  from 
a  dicotyledonous  ancestry.  The  justice  of  this  hypothesis,  how- 
ever, will  be  finally  established  only  when  we  shall  have  at  our 
disposal  for  anatomical  investigation  remains  of  monocotyledons 
from  Mesozoic  deposits.  Whether  or  not  it  is  ultimately  estab- 
lished that  the  large  group  at  present  under  discussion  has  actually 
been  derived  from  the  dicotyledons,  it  will  doubtless  in  any  case 
be  clear  that  they  cannot  in  any  way  be  regarded  as  primitive 
representatives  of  the  angiosperms.  The  monocotyledons  in 
fact  represent  the  herbaceous  type  in  its  extremest  form.  In 
the  group  the  fibrovascular  tissues  are  released  from  the  rigid 
confinement  of  the  tubular  stele  in  the  ancestral  forms  with  sec- 
ondary growth  by  the  development  of  large  parenchymatous 
storage  devices  in  relation  to  the  foliar  traces.  Correlated  with 
this  release  is  the  possibility  of  accommodation  of  the  numerous 
foliar  strands  which  characterize  the  basal  regions  of  monocotyledo- 
nous leaves  throughout  the  transverse  section  of  the  stem.  There 
is  doubtless  some  correlation  also  between  the  extreme  multipli- 
cation and  consequent  displacement  of  the  strands  in  the  stem 
of  monocotyledons  and  the  disappearance  of  secondary  growth. 
Possibly  an  aquatic  or  amphibious  habitat  long  maintained  may 
likewise  have  acted  as  a  contributory  cause  in  bringing  about  the 
obliteration  of  cambial  activity,  since  in  the  dicotyledonous 
Nymphaceae,  in  which  there  is  diffuse  distribution  of  the  bundles 
and  also  the  absence  of  cambial  activity,  we  find  these  features 
correlated  with  an  aquatic  habitat.  It  has,  indeed,  often  been 
suggested  that  the  Nymphaceae  or  Ranunculaceae  are  the  dicoty- 
ledonous ancestors  of  the  monocotyledons.  The  interesting  inves- 
tigations of  Sargent  on  the  fusion  of  the  cotyledonary  structures 
in  Ranunculus  ficaria,  etc.,  are  of  importance  in  indicating  how 
monocotyledony  may  have  arisen  as  a  result  of  the  union  of  two 
originally  separate  seed  leaves.  Another  possibility,  of  course,  is 
the  origin  of  the  monocotyledonous  embryo  as  a  consequence  of 
the  abortion  of  one  of  the  two  original  cotyledons,  and  this  view 
is  perhaps  supported  by  the  conditions  found  in  certain  grasses, 
such  as  Zizania,  Avena,  etc.,  in  which  the  vestige  of  a  second 
cotyledon  is  considered  to  be  present.  The  problem  of  the  origin 


416  THE  ANATOMY  OF  WOODY  PLANTS 

of  the  monocotyledons  cannot  yet  be  regarded  as  by  any  means 
settled,  and  a  much  fuller  knowledge  of  the  anatomy  of  extinct 
and  living  representatives  of  this  extremely  important  group  is 
necessary  before  any  final  results  can  be  reached.  If  we  attempt 
to  picture  to  ourselves  the  probable  future  course  of  evolution  in 
the  angiosperms,  it  is  difficult  to  concede  to  the  monocotyledons 
a  prominent  position.  This  group  seems  to  have  reached  such  an 
extreme  degree  of  specialization  that  it  will  probably  in  the  long 
run  entirely  disappear  and  be  replaced  by  new  derivatives  of  the 
still  plastic  dicotyledons. 


CHAPTER  XXX 
ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION 

General  views  in  regard  to  the  ancient  climatic  conditions 
must  first  occupy  attention  in  the  present  connection.  It  is 
commonly  conceded,  on  the  basis  both  of  the  nature  of  ancient 
organisms  and  of  the  evidence  supplied  by  geologic  strata,  that 
the  earth  was  formerly  much  warmer  than  it  is  in  the  present 
epoch.  It  is  further  clear  that,  other  things  being  equal,  the 
greatest  degree  of  warmth  existed  in  the  most  remote  past.  This 
general  situation,  however,  does  not  exclude  the  recurrence  at 
long  intervals  of  periods  of  refrigeration  or  glaciation.  An  age  of 
ice  is  known  to  have  occurred,  not  only  at  the  end  of  the  Cenozoic 
as  originally  established  by  Agassiz,  but  glacial  periods  also  termi- 
nated both  the  Mesozoic  and  the  Paleozoic  as  ordinarily  denned. 
Evidence  of  still  earlier  glacial  epochs  which  exercised  a  devastating 
influence  on  the  most  ancient  animal  and  plant  populations  of 
our  earth  is  not  lacking. 

Evidence  in  regard  to  glaciation  in  former  epochs  is  both  direct 
and  indirect.  Direct  testimony  concerning  former  ages  of  ice 
is  supplied  by  the  comparative  study  of  deposits  formed  in  con- 
nection with  the  existing  glaciers  of  high  latitudes  or  of  high 
altitudes.  Intimately  connected  with  glacial  phenomena  are  the 
formations  of  clays,  till,  and  coarse  morainal  matter  resulting 
from  the  movement  and  melting  of  ice.  Indications  of  the  kinds 
just  enumerated  in  earlier  geological  strata  supply  direct  testimony 
as  to  former  glacial  action.  Indirect  information  in  the  same 
direction  is  often  furnished  by  the  wholesale  extinction  of  impor- 
tant groups  of  plants  and  animals.  For  example,  in  the  Permian 
glaciation  which  marked  the  close  of  the  Paleozoic  the  treelike 
cryptogams,  which  have  contributed  so  largely  to  the  formation 
of  the  older  deposits  of  combustible  minerals,  disappeared  entirely 
as  an  important  constituent  of  the  plant  population  of  our  earth. 
Glacial  epochs  are,  however,  not  of  direct  importance  in  relation 

417 


4i8  THE  ANATOMY  OF  WOODY  PLANTS 

to  the  evolution  of  plants  in  response  to  climatic  influences,  since 
their  action  is  mainly  negative.  It  is  true,  however,  that  by 
bringing  about  the  obliteration  of  important  groups  of  plants  or 
animals  greater  opportunity  is  supplied  for  the  surviving  and  more 
adaptable  forms  to  develop  in  the  following  warmer  epochs. 

It  is  the  more  gradual  and  not  the  spasmodic  refrigeration 
which  has  produced  perhaps  the  greatest  effect  on  the  organization 
of  the  successive  plant  populations  of  the  earth.  As  a  preliminary 
to  the  discussion  of  the  anatomical  modifications  which  are  more 
or  less  definitely  correlated  with  climatic  changes  in  the  successive 
geological  ages,  the  evidence  furnished  by  plants  in  regard  to 
progressive  climatic  cooling  must  be  considered.  This  evidence 
is  of  two  kinds.  Perhaps  the  most  important  and  trustworthy 
is  derived  from  the  organization  of  the  secondary  woods  in  trees 
of  the  various  geological  periods.  This  testimony  can  hardly 
be  estimated  too  highly  in  arriving  at  any  conclusions  in  regard 
to  plants  as  reliable  indicators  of  climatic  -change.  Another 
kind  of  evidence  is  afforded  by  the  character  of  the  plants  them- 
selves. At  the  present  time  there  are  large  groups  of  plants  which 
are  of  more  or  less  definite  tropical  occurrence  and  others  which 
prevail  characteristically  in  cooler  regions.  The  advance  of 
tropical  types  toward  the  poles  or  the  progress  of  polar  plants 
in  the  direction  of  the  equator  in  earlier  geological  eras  must, 
other  things  being  equal,  indicate  variations  in  climatic  conditions 
in  the  direction  either  of  greater  or  of  less  warmth,  as  the  case 
may  be.  Unfortunately,  in  earlier  geologic  times  the  differen- 
tiation between  polar  and  equatorial  types  was  not  nearly  so 
marked  as  it  is  at  present,  and  this  general  situation  militates 
more  or  less  strongly  against  reliable  conclusions  in  regard  to 
climate  in  all  but  the  latest  geological  eras.  Since  the  present 
work  deals  with  anatomy,  the  subject  of  plant  geography  in  rela- 
tion to  the  climatic  changes  which  have  marked  the  successive 
ages  of  the  earth  is  obviously  of  less  importance,  aside  from  the 
limitations  indicated  above. 

As  has  been  already  shown,  the  organization  of  the  secondary 
wood  in  extinct  plants  furnishes  the  most  reliable  evidence  as 
to  the  climatic  conditions  which  prevailed  in  earlier  geological 


ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION    419 

epochs.  The  information  derived  from  this  source  is  not  only 
on  the  whole  the  most  trustworthy,  but  it  is  likewise  present  in 
sufficient  quantity 
from  different  geo- 
logical periods  and 
diverse  geographic 
areas  to  justify 
more  or  less  com- 
prehensive general 
conclusions.  In 
the  Paleozoic 
trunks  which  are 
supplied  by  the 
geological  forma- 
tions of  Southern 
Canada  the 
organization  of  the 
wood  shows  great 
Uniformity,  and  FIG.  287.— Transverse  section  of  the  wood  of  Cor- 

there      are     no      daites  from  Prince  Edward  Island. 

modifications  of 
structure  which 
indicate  any  perio- 
dicity in  annual  con- 
ditions of  growth. 
The  truth  of  this 
statement  is  well 
illustrated  by  the  ac- 
companying photo- 
graph (Fig.  287)  of 
the  wood  of  a  cordai- 
tean  form  from  the 
Permo- Carbonifer- 
ous of  Hampton, 
Prince  Edward 

FIG.  288.— Transverse  section  of  the  wood  of  Cor-     ^^n&-     The   pres- 
daites  from  the  north  of  England.  ence  of  clear    zones 


420 


THE  ANATOMY  OF  WOODY  PLANTS 


of  periodic  growth  is,  however,  frequently  found  in  regions  of  higher 
latitude  than  that  portrayed  in  Fig.  288.  The  next  illustration 
shows  the  organization  of  a  Carboniferous  cordaitean  wood  (Mesoxy- 
lon)  from  the  northern  part  of  England  and  consequently  of  consider- 


FIG.  289. — Details  of  wood  structure  of  Mesoxylon  (Cordaites)  with  annual  rings. 
Tangential  pitting  is  absent. 

ably  higher  latitude  (54°  N.  in  contrast  to  the  46°  N.,  the  latitude  of 
Prince  Edward  Island).  The  annual  rings  in  the  wood  from  the 
English  Carboniferous  are  clearly  marked.  The  next  illustration 
(Fig.  289)  reproduces  the  minute  organization  of  the  same  wood 


ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION    421 

under  a  sufficient  degree  of  magnification  to  show  the  essential  fea- 
tures of  structure.  The  tracheids  are  so  long  that  only  a  small 
portion  of  them  can  be  shown  in  the  figure.  Their  walls  are 
covered  with  crowded  pits  arranged  in  an  alternating  fashion  with 
no  intervening  bars  of  Sanio.  The  only  parenchyma  present  is 
radial,  and  the  end  of  the  annual  ring  is  marked  by  the  presence 
of  tracheids  which  are  slightly  narrower  in  radial  dimensions  but 
do  not  otherwise 
differ  from  those 
found  in  the  rest  of 
the  annual  ring. 

For  comparison 
with  the  situation 
revealed  by  the 
cordaitean  wood 
from  Northern 
England,  a  trunk 
from  the  Triassic 
of  the  southwest 
region  of  the 
United  States  is 
shown.  In  Fig.  290 
appears  a  trans- 
verse section  of  the 
wood  of  a  tree  from 
the  Triassic  forest 

of  Arizona.  The  annual  rings  are  not  so  distinct  in  the  photo- 
micrograph as  they  appear  on  the  weathered  end  of  the  actual 
petrified  specimen.  It  will  be  clear  from  the  information  sup- 
plied in  this  case  that  as  far  south  as  Arizona  in  the  Triassic 
annual  rings  were  more  or  less  clearly  marked.  A  noteworthy 
variation  in  the  annual  temperature  in  that  somewhat  remote 
epoch  is  thus  indicated.  This  situation  presents  an  interesting 
contrast  to  the  climatic  conditions  which  prevailed  in  the  region 
of  Prince  Edward  Island  toward  the  end  of  the  Paleozoic.  If  the 
situation  be  summarized,  it  is  clear  that  in  the  later  Paleozoic 
the  difference  between  46°  N.  and  54°  N.  means  the  presence  in  the 


FIG.  290. — Transverse  section  of  a  coniferous  wood 
from  the  Trias  of  Arizona. 


422  THE  ANATOMY  OF  WOODY  PLANTS 

higher  latitude  of  annual  rings  and  their  absence  in  the  lower  one. 
On  the  other  hand,  in  the  beginning  of  the  Mesozoic  (the  Triassic) 
even  at  a  distance  of  10  degrees  south  of  the  latitude  of  Prince 
Edward  Island  annual  rings  were  quite  clearly  developed. 

Without  considering  the  Jurassic  we  may  pass  at  once  to 
the  end  of  the  Mesozoic  period,  namely,  the  Upper  Cretaceous. 
Fig.  291  reproduces  the  organization  of  the  wood  of  a  Cretaceous 

araucarian  conifer 
closely  allied  to,  if 
not  identical  with, 
the  living  kauri 
(Agathis).  A  per- 
sistent leaf  trace 
appears  running 
through  the  figure 
transversely.  The 
wood  is  considerably 
over  half  a  century 
(as  recorded  in 
annual  rings)  from 
the  center  of  the 
stem,  and  the  foliar 
strand  still  persists 
in  the  manner 

FIG.  291. — Transverse  section  of  an  Araucanoxylon  .     . 

from  the  Cretaceous  of  state*  island.  characteristic  of  re- 

lated living  arau- 
carian genera.  The  yearly  increments  of  growth  are  distinctly 
indicated,  although  not  very  strongly  marked.  For  comparison 
with  the  illustration  above  a  photograph  of  the  wood  of  Agathis 
australis  from  New  Zealand  is  shown  in  Fig.  292.  Here  annual 
rings  are  clearly  apparent,  although  the  latitude  in  the  Southern 
Hemisphere  is  nearly  the  same  as  the  place  of  origin  of  the 
Cretaceous  araucarian  wood  above,  namely,  40  degrees.  It  is 
thus  apparent  that  the  Cretaceous  of  Staten  Island  was  marked 
by  a  much  less  distinct  annual  variation  of  temperature  than 
is  the  South  Island  of  New  Zealand  of  today,  although  the  two 
localities  are  of  almost  identical  latitude.  Without  pursuing  the 


ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION     423 


subject  further  we  may  point  out  that  the  organization  of  the 
trunks  of  trees  in  succeeding  geological  epochs  indicates  a  gradual 
cooling  of  terrestrial  climates  which,  when  it  became  accentuated, 
gave  rise  to  a  marked  variation  in  annual  temperature,  recorded 
ever  more  clearly  in  the  progressively  greater  definition  of  the 
annual  rings  in  later  geological  times. 

It  is  now  necessary  to  correlate  the  climatic  changes  more  in 
detail  with  the 
internal  organiza- 
tion of  the  plastic 
stem  organs  of 
vascular  plants. 
It  has  been  shown 
in  Fig.  289,  repre- 
senting the  wood 
of  the  cordaitean 
Mesoxylon  of  Car- 
boniferous age 
from  the  north  of 
England,  that  the 
only  differentiation 
which  marked 
annual  increments 
of  growth  in  this 
early  age  was  a 
slight  narrowing 

in  radial  diameter  of  the  tracheids  formed  toward  the  end  of  the 
period  of  growth.  No  other  structural  feature  related  to  the 
phenomenon  of  annual  rings  has  been  recorded  as  yet  from  either 
the  late  Paleozoic  or  from  the  early  Mesozoic  (Triassic).  In  the 
Jurassic,  however,  and  practically  universally  from  this  epoch 
onward,  a  marked  modification  in  the  organization  of  the  annual 
rings  presents  itself.  This  is  well  shown  in  the  structure  of  the 
wood  of  Ginkgo  (Fig.  293),  a  survivor  of  a  stock  which  attained  its 
zenith  of  development  in  the  Mesozoic  age.  Clearly  the  tracheids 
which  mark  the  termination  of  the  annual  increment  differ  from 
those  previously  formed  in  the  character  of  their  pitting.  The 


FIG.  292. — Transverse  section  of  the  wood  of  Agatkis 
australis  from  New  Zealand. 


424 


THE  ANATOMY  OF  WOODY  PLANTS 


pores  of  the  tracheary  elements  in  the  first-formed  region  of  the 
annual  ring  are  exclusively  radial.  The  terminal  tracheids  in  con- 
trast are  distinguished  by  the  presence  of  tangential  pitting.  It  is 
in  the  gymnosperms  of  the  middle  and  later  Mesozoic  that  this 


FIG.  293. — Details   of  organization   of   wood  of   Gink  go.    Tangential   pitting 
appears  at  the  end  of  the  annual  rings. 

feature  is  first  clearly  expressed.  It  has  been  suggested  by  Stras- 
burger  that  tangential  pitting  is  a  device  for  supplying  water 
rapidly  and  abundantly  in  the  new  period  of  growth  to  the  reawak- 
ening cambium.  This  seems  to  be  a  highly  probable  explanation 
of  the  anatomical  conditions  present  and  may  be  accepted  in  the 


ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION    425 

absence  of  any  evidence  against  its  validity.  The  gymnosperms 
of  the  Paleozoic  had  just  begun  to  develop  annual  rings  toward  the 
end  of  the  epoch,  and  the  phenomenon  of  tangential  pitting  had  not 
yet  made  its  appearance.  As  has  been  illustrated  above,  this  feature 
did  not  become  marked  until  the  Mesozoic  was  well  advanced. 


FiG.  294. — Details  of  organization  of  the  wood  in  Picea  canadensis 

The  situation  in  gymnospermous  woods  did  not,  however, 
rest  with  the  appearance  of  tangential  pitting  at  the  end  of  the 
annual  ring.  Not  long  after  this  feature  had  become  well  estab- 
lished it  was  accompanied  by  another  even  more  important  ana- 
tomical phenomenon.  In  Fig.  294  is  shown  a  slightly  diagrammatic 
view  of  the  wood  of  the  root  of  Picea  canadensis  in  three  dimen- 
sions. The  illustration  includes  the  transition  from  one  annual 
ring  to  the  next,  and  the  terminal  tracheids  manifest  the  tangential 


426  THE  ANATOMY  OF  WOODY  PLANTS 

pitting  referred  to  above.  Not  only  is  the  end  of  the  annual 
increment  marked  by  tangential  pitting,  but  it  is  also  distinguished 
by  the  presence  of  longitudinal  tangential  storage  elements  known 
as  wood  parenchyma.  In  the  figure  these  living  cells  are  strictly 
confined  to  the  face  or  termination  of  the  annual  ring.  They  are 
further  present  as  derivatives  from  tracheary  structures,  for 
stages  between  them  and  transversely  divided  tracheids  are  plainly 
seen.  Both  the  mode  of  occurrence  and  the  manner  of  origin  of 
the  first  parenchyma tous  elements  in  coniferous  woods  are  of 
equal  interest  from  the  climatological  and  evolutionary  standpoints. 
The  position  of  the  tangential  storage  elements  at  the  end  of  the 
annual  ring  corresponds  in  an  apparently  highly  significant  manner 
with  the  appearance  of  tangential  pitting  in  those  woods  of  later 
geological  time  which  have  developed  the  phenomenon  of  annual 
rings.  It  has  been  suggested  with  a  strong  degree  of  probability 
that  tangential  pitting  in  the  gymnosperms  is  an  adaptation  for 
supplying  abundance  of  water  to  the  cambium  when  it  renews  its 
activity  after  the  annual  period  of  rest.  It  is  extremely  likely 
that  the  terminal  parenchyma  which,  so  far  as  our  present  knowl- 
edge goes,  first  made  its  appearance  in  the  Jurassic  is  likewise 
a  device  favoring  the  activity  of  the  cambium  in  a  new  period 
of  growth.  If  it  is  probable  that  the  tangential  pitting  of  the 
terminal  tracheids  is  for  the  purpose  of  supplying  the  cambium 
with  water,  it  seems  not  less  likely  that  terminal  parenchyma 
provides  a  convenient  supply  of  food  for  the  initial  cells  of  the 
cambial  layer.  The  conditions  present  strongly  suggest  that  the 
first  appearance  of  tangential  storage  elements  was  in  relation  to 
the  nutritive  demands  of  the  cambial  layer.  Whether  that  con- 
clusion is  accepted  or  not,  there  can  be  no  doubt  of  the  correlation 
of  the  phenomenon  of  annual  rings  and  the  first  appearance  of 
longitudinal  parenchyma  in  the  tissues  of  the  wood.  This  view 
of  the  matter  is  strongly  supported  by  the  conditions  observed 
in  Paleozoic  woods,  for  these  are  equally  characterized  by  the 
absence  of  annual  rings  (except  of  course  at  the  end  of  the  period 
and  in  higher  latitudes)  and  of  storage  elements  belonging  to  the 
category  of  wood  parenchyma. 

Although  at  first  terminal  in  position  in  gymnospermous  woods, 
parenchyma  did  not  long  remain  restricted  to  this  situation.     In 


ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION    427 

the  Jurassic  and  Cretaceous,  storage  elements  are  already  found 
distributed  throughout  the  annual  ring.  Among  the  living  conif- 
erous subtribes  all  but  the  Abietineae  have  or  formerly  possessed 
diffuse  parenchyma  or  storage  tissue  scattered  throughout  the 
annual  ring.  This  condition,  for  example,  actually  holds  for  the 
Taxodineae,  Cupressineae,  and  Podocarpineae,  and  there  is  good 
evidence  either  from  fossil  forms  or  conservative  organs  or  from 
both  together  that  the  Taxineae  and  Araucariineae  formerly 
possessed  abundant,  diffuse  storage  parenchyma.  In  the  Abie- 
tineae a  very  interesting  condition  exists  which  is  quite  in 
harmony  with  the  ancestral  position  assigned  to  them  in  an  earlier 
chapter.  In  the  group  Pineae  parenchymatous  elements  are 
absent  in  the  ancient  Pinus  and  Prepinus;  while  in  Picea,  Larix, 
and  Pseudotsuga  they  occur  in  an  exclusively  terminal  position 
and  present,  particularly  in  Picea  (notably  in  the  root  of  the  various 
species),  all  transitionary  stages  of  derivation  from  septate  tra- 
cheids.  It  may  be  assumed  that  in  the  Pineae  we  have  the  most 
primitive  conditions  found  in  the  conifers  as  regards  the  presence, 
position,  and  mode  of  origin  of  parenchymatous  elements.  In  the 
Abieteae,  comprising  Abies,  Tsuga,  Cedrus,  and  Pseudolarix,  the 
longitudinal  storage  parenchyma  is  characteristically  terminal, 
but  no  longer  shows  normal  evidence  of  derivation  from  tracheary 
elements.  In  wood  formed  after  injury,  however,  transitions 
from  tracheids  to  parenchyma  cells  can  readily  be  observed.  The 
genera  Abies  and  Tsuga  are  of  particular  interest  among  the 
Abieteae  on  account  of  the  fact  that  some  of  their  species  show 
a  transition  toward  the  diffuse  condition  of  distribution  of  wood 
parenchyma.  In  A .  webbiana  and  A .  cephalonica  storage  elements 
scattered  throughout  the  annual  ring  have  been  observed,  and  a 
similar  statement  holds  for  the  mature  wood  of  T.  mertensiana 
and  the  branches  of  T.  canadensis.  In  the  remaining  subtribes 
of  conifers,  as  shown  above,  the  parenchyma  is  or  has  been  diffuse 
in  its  mode  of  occurrence.  Further,  it  presents  no  normal  tran- 
sitions from  tracheary  elements,  although  such  conditions  can 
frequently  be  observed  in  injured  material. 

The  conifers  have  been  chosen  to  illustrate  the  correlation 
between  the  organization  of  wood  and  climatic  conditions," 
because  their  great  geological  age  and  the  abundance  of  their 


428  THE  ANATOMY  OF  WOODY  PLANTS 

structurally  preserved  fossil  woods  make  them  a  particularly 
significant  document  in  this  connection.  In  the  higher  living 
gymnosperms  and  in  the  dicotyledonous  angiosperms  the  diffuse 
distribution  of  the  longitudinal  storage  elements  is  that  typically 
present.  It  is  reasonable  to  suppose,  in  the  absence  of  the  abun- 
dant evidence  presented  in  the  case  of  the  Coniferales,  that  in 
the  remaining  living  seed  plants  wood  parenchyma  originally 
appeared  much  in  the  same  manner  as  in  the  coniferous  gymno- 
sperms. 

It  has  been  noted  in  a  previous  chapter  that,  although  in  the 
mass  of  conifers  tangential  pitting  is  characteristic  of  the  terminal 
tracheids  of  the  annual  ring  alone,  in  the  higher  groups  of  seed 
plants  it  is  diffused  throughout  the  annual  ring  precisely  as  is 
the  case  with  the  wood  parenchyma.  It  is  thus  apparent  that 
the  phenomenon  of  annual  rings  correlated  to  the  recurrence  of 
yearly  periods  unfavorable  to  vegetative  activity  is  in  turn  cor- 
related to  the  appearance  of  tangential  pitting  and  wood  paren- 
chyma in  the  series  presented  by  the  more  modern  seed  plants. 
It  thus  becomes  evident  that  climatic  conditions,  so  far  as  these 
two  extremely  important  features  of  organization  of  the  wood  are 
concerned,  have  had  a  potent  influence  on  the  organization  of 
structure. 

The  next  impetus  to  the  upward  evolution  is  provided  by  the 
appearance  of  vessels.  It  is  highly  improbable  that  these  impor- 
tant structural  features  of  the  wood  in  higher  plants  owe  their 
origin  in  any  way  to  climatic  conditions.  Their  appearance, 
although  a  phenomenon  of  the  highest  importance  from  the  stand- 
point of  the  doctrine  of  descent,  is  not  obviously  connected  with 
any  known  causal  conditions.  There  is  no  question,  however, 
that  the  histological  element  known  as  the  vessel  has  come  into 
existence  as  a  consequence  of  the  modification  of  tracheids  of 
the  secondary  wood.  This  is  the  situation  in  the  case  of  the 
Gne tales  and  is  only  less  distinctly  manifested  by  the  lower  dicot- 
yledons. Although  the  appearance  of  vascular  elements  has 
no  relation  to  climatic  conditions,  vessels  once  established  as  a 
feature  of  ligneous  organization  have  played  an  important  part 
'  in  the  evolutionary  history  of  the  higher  vascular  plants  in  rela- 


ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION    429 

tion  to  conditions  of  temperature.  The  secondary  wood  of  the 
higher  plants  is  much  more  efficient  in  conducting  water  when 
vessels  are  present.  It  has  been  estimated  by  Pfeffer,  for  example, 
that  the  woody  cylinder  of  the  birch  has  about  twenty  times  the 
water-conducting  capacity  of  that  of  the  pine,  and  the  second- 
ary xylem  of  the  Betulaceae  is  probably  less  viable  to  water,  on 
account  of  the  universal  presence  of  vessels, with  scalariform  per- 
forations, than  are  many  dicotyledonous  woods  of  higher  organiza- 
tion with  porously  perforated  vessels.  The  greater  capacity  for 
conducting  water  which  characterizes  the  secondary  wood  of 
seed  plants  from  the  Gnetales  upward  provides,  other  things  being 
equal,  for  a  much  greater  leaf  surface.  The  greater  superficial 
development  of  foliar  organs  naturally  results  in  a  much  increased 
assimilative  capacity.  The  greater  accumulation  of  the  products 
of  photosynthesis  directly  consequent  on  increase  of  foliar  surface, 
and  indirectly  on  the  appearance  of  vessels  as  an  important  feature 
of  structure  in  the  wood,  naturally  involves  an  increase  of  storage 
capacity.  Since  the  secondary  wood  is  the  most  important  storage 
tissue  of  the  higher  plants,  increased  accommodation  for  reserve 
materials  appropriately  originates  as  a  result  of  modification  of 
its  structure.  As  has  been  pointed  out  in  earlier  chapters,  the 
Gnetales  present,  not  only  the  lowest  occurrence  of  vessels  in 
secondary  wood,  but  at  the  same  time  the  first  appearance  of 
the  typical  aggregate  ray.  This  type  of  radial  storage  device 
results  from  the  clustering  and  partial  fusion  of  the  original  narrow 
rays.  Aggregate  rays  were  at  first  not  specially  related  to  the 
appendages,  but  in  higher  types  tend  to  become  somewhat  definitely 
connected  with  leaves,  roots,  and  other  lateral  organs.  The 
aggregate  ray  passes  by  further  changes  into  the  compound  ray, 
characterized  by  a  homogeneous  organization  and  no  longer  includ- 
ing fibers  and  other  features  of  longitudinal  structure  of  the  wood. 
As  an  alternative  to  the  compound  ray  which  is  the  result  of  fusion, 
we  have  the  diffuse  condition  of  radial  parenchyma  resulting  from 
the  divergence  of  the  original  clusters  or  aggregations  of  rays  in 
the  outer  regions  of  the  woody  cylinder.  We  have  accordingly 
to  do  with  two  derivatives  of  the  aggregate  ray:  the  compound 
condition  resulting  from  fusion  and  the  diffuse  modification  which 


430  THE  ANATOMY  OF  WOODY  PLANTS 

is  the  consequence  of  the  separation  of  the  original  components  of 
the  congeries  or  aggregation  of  rays. 

The  diffuse  type  of  radial  organization  appears  to  have  been 
more  in  accordance  with  the  needs  of  arboreal  dicotyledons  in 
later  geological  times  in  which  climatic  refrigeration,  annual  or 
secular,  has  become  more  and  more  marked.  It  is  practically 
of  universal  occurrence  in  the  forest  trees  of  the  present  epoch. 
The  compound  type  of  ray  is  limited  to  a  very  few  species  of  trees, 
of  which  the  oak  is  a  notable  example. 

Although  large  rays  of  the  compound  type  are  usually  entirely 
absent  in  arboreal  forms,  they  are  extremely  common  in  herbaceous 
stems  belonging  to  diverse  and  not  nearly  allied  families  of  dicoty- 
ledons. The  accumulation  of  large  quantities  of  storage  tissues 
about  the  foliar  traces,  a  disadvantageous  condition  in  deciduous 
trees,  is  very  frequent  in  typical  herbs  and  vines  of  herbaceous 
texture.  There  is  an  interesting  correlation  between  climatic 
conditions  and  the  presence  of  herbaceous  types  which  is  not 
only  geographical,  but  likewise  geological.  Taking  the  geographical 
or  climatological  conditions  first,  we  know  that  it  is  a  notable  fact 
that  arboreal  forms  are  much  more  prevalent  within  the  tropics 
than  they  are  in  temperate  regions.  This  statement  naturally 
applies  to  the  higher  rather  than  to  the  lower  families  of  the 
dicotyledons.  In  the  true  amentiferous  types,  such  as  the  Betu- 
laceae,  Fagaceae,  Juglandaceae,  etc.,  there  are  no  herbaceous 
representatives  at  all,  although  these  families  are  characteristically 
distributed  in  temperate  climates.  The  absence  of  herbaceous 
forms  in  this  instance  presents  an  interesting  resemblance  to  the 
conditions  found  among  the  Coniferales,  which  are  also  without 
species  of  herbaceous  texture,  even  in  the  extreme  polar  limits 
of  their  range.  The  facts  in  the  two  parallel  cases  seem  to  have 
a  common  and  significant  explanation,  which  is  that  both  are 
relatively  primitive  groups.  It  is  in  the  higher  orders,  such  as 
the  Compositae,  Solanaceae,  Leguminosae,  etc.,  that  the  principle 
of  the  occurrence  of  herbaceous  types  in  temperate,  and  of  arboreal 
forms  in  tropical,  climates  is  well  illustrated.  There  are,  for 
example,  very  few  leguminous  trees  in  north-temperate  regions, 
while  herbaceous  forms  are  extremely  common.  Contrariwise, 


ANATOMICAL  STRUCTURE  AND  CLIMATIC  EVOLUTION    431 

tropical  arboreal  Leguminosae  are  common,  while  herbs  belonging 
to  this  family  are  much  less  abundant  in  equatorial  latitudes. 

The  geological  conditions  so  far  as  they  are  clearly  displayed 
seem  to  correspond  closely  with  those  indicated  geographically  in 
the  present  era.  For  example,  the  Paleozoic,  which  was  a  period 
of  average  high  temperature  compared  with  later  geological  eras, 
was  characterized  by  the  prevalence  of  arboreal  cryptogams  belong- 
ing to  the  groups  whose  survivors  are  practically  entirely  herba- 
ceous. In  the  cooler  but  still  warm  Mesozoic  the  prevailing  type 
of  seed  plant  vegetation  was  gymnospermous  and  in  particular 
coniferous,  and  was  consequently  arboreal  and  not  herbaceous  in 
habit.  Concerning  the  proportion  of  herbs  and  trees  in  the 
angiospermous  vegetation  of  the  later  Mesozoic  and  of  the  warmer 
part  of  the  Tertiary  we  are  unfortunately  not  well  informed.  It 
is  certain  from  still  unpublished  investigations  that  herbaceous 
types  were  present,  although  not  abundant,  in  the  Upper  Cretaceous 
of  the  Eastern  United  States.  As  the  data  in  this  case  are  sup- 
plied by  remains  preserved  from  obliteration  by  charring  and 
subsequent  sweeping  into  open  water,  it  is  not  unlikely  that  they 
indicate,  approximately  at  any  rate,  the  proportion  of  herbaceous 
and  woody  forms  in  the  floras  of  the  later  Cretaceous. 

The  general  evidence  points  conclusively  toward  the  herb  as  a 
product  of  cooler  climatic  conditions  and  of  later  geological  times. 
It  should  not  be  forgotten  in  this  connection  that  the  appearance 
of  wood  parenchyma,  with  which  in  the  last  analysis  the  evolution 
of  herbaceous  angiosperms  is  clearly  linked,  is  definitely  related 
to  the  phenomenon  of  annual  rings,  in  turn  the  result  of  climatic 
refrigeration  in  later  geological  epochs.  The  impulse  toward  the 
formation  of  tangential  parenchyma  was  thus  obviously  supplied 
by  climatic  cooling.  Tangential  parenchyma  united  with  radial 
storage  tissues  furnishes  the  explanation  of  the  broad  or  compound 
ray  as  exemplified  by  the  oak  on  the  one  hand  and  by  vines  and 
herbs  on  the  other.  It  will  be  apparent  from  the  conditions 
elucidated  above  that  climate  has  had  a  paramount  influence  in 
molding  the  organization  of  plants  and  that  refrigeration  has 
always  favored  the  appearance  of  herbaceous  types.  A  careful 
distinction  must,  however,  be  made  between  degenerate  herbs 


432  THE  ANATOMY  OF  WOODY  PLANTS 

and  dynamic  ones.  Exemplifications  of  the  former  are  supplied 
by  the  few  surviving  vascular  cryptogams.  The  latter  are  rep- 
resented by  the  huge  aggregation  of  angiospermous  herbs,  both 
dicotyledonous  and  monocotyledonous,  which  constitute  such  a 
preponderant  and  aggressive  element  in  the  present  plant  popula- 
tion of  our  earth.  It  seems  not  unlikely  that  should  present 
climatic  conditions  remain  for  a  long  period  either  unchanged  or 
even  accentuated,  the  dynamic  herbaceous  type  will  definitely 
supplant  the  arboreal,  at  least  in  temperate  regions.  If  this  view 
is  correct,  we  must  regard  the  strikingly  herbaceous  flora  of  exist- 
ing prairies,  steppes,  and  pampas  of  higher  latitudes  as  a  fore- 
runner of  a  condition  which  will  ultimately  become  universal. 
In  any  case  the  appearance  of  the  dynamic  herb  of  angiospermous 
affinities,  which  is  primarily  the  result  of  the  differentiation  and 
not  the  degeneracy  of  the  woody  cylinder,  is  an  evolutionary 
phenomenon  of  the  first  order. 


CHAPTER  XXXI 
EVOLUTIONARY  PRINCIPLES  EXHIBITED  BY  THE  COMPOSITAE 

In  earlier  pages  the  principles  derived  from  the  comparative 
anatomical  study  of  existing  and  extinct  plants,  particularly 
vascular  cryptogams  and  gymnosperms,  have  been  emphasized. 
In  the  present  chapter  it  will  be  shown  that  these  principles  are 
as  applicable  to  forms  the  past  of  which  is  unknown  as  to  those 
historically  recorded.  It  will  be  advantageous  in  this  connection 
to  consider  a  very  high  group  among  the  dicotyledons,  namely, 
the  Compositae.  This  family  is  commonly  divided  into  two  sec- 
tions, the  Tubuliflorae  and  the  Liguliflorae.  The  former  are 
characterized  by  the  fact  that  the  axial  florets  of  the  heads  are 
tubular  and  not  ligulate  in  their  organization.  Anatomically 
they  are  distinguished  by  the  presence  of  oil  canals,  which  usually 
occur  in  the  more  conservative  organs  even  when  they  are  lacking 
elsewhere.  In  the  Liguliflorae,  on  the  other  hand,  the  axial  flowers 
of  the  inflorescence  are  invariably  ligulate.  Histologically  this 
group  is  characterized  by  a  milky  juice  which  is  present  in  the 
pericycle  and  the  phloem.  Indeed,  it  often  happens  that  the 
laticiferous  ducts  are  actual  sieve  tubes  or  are  at  least  continuous 
with  elements  of  this  nature.  It  is  an  interesting  fact  that  in 
the  higher  representatives  of  the  Compositae,  marked  by  the 
presence  of  a  milky  or  laticiferous  secretory  system,  frequently 
oil  canals  such  as  occur  normally  in  the  Tubuliflorae  are  present 
under  the  same  conditions  as  the  ancestral  features  surviving  in 
vascular  cryptogams  and  gymnosperms.  This  situation  is  of 
great  general  interest  as  illustrating  the  wide  validity  of  the  general 
principles  elucidated  in  chapter  xvii.  Since  the  Compositae  are 
readily  obtainable  by  reason  of  their  great  abundance  in  the 
present  flora,  either  in  a  wild  or  in  a  cultivated  state,  they  may 
serve  advantageously  to  illustrate  pedagogically  the  fundamental 
conceptions  of  comparative  anatomy. 

In  Fig.  295  is  shown  a  transverse  section  of  part  of  the 
stem  of  the  Jerusalem  artichoke  (Helianthus  tuberosus)  moderately 

433 


434 


THE  ANATOMY  OF  WOODY  PLANTS 


FIG.  295. — Transverse  section  of  part  of  stem  of  the  Jerusalem  artichoke 


FIG.  296. — Section  of  part  of  the  root  of  a  species  of  Aster 


EVOLUTIONARY  PRINCIPLES  OF  THE  COMPOSITAE      435 


magnified.  In  the  cortex,  lying  outside  the  series  of  bundles,  may 
be  seen  two  or  three  circular  spaces  surrounded  by  secretory  epithe- 
lium. These  are  the  oil  canals  of  the  tubuliflorous  section  of  the 
Compositae.  Smaller  canals  of  the  same  type  can  less  readily 
be  distinguished  in  the  medullary  region.  Such  oil  canals  are 
found  quite  generally  in  the  Tubuli- 
florae  and  are  rarely  absent  in  the  root, 
even  when  they  become  abortive  in 
other  regions.  Fig.  296  illustrates  the 
occurrence  of  similar  canals  in  the  root 
of  Aster.  In  this  genus  the  canals  are 


•^>u  >^~~  w^  v^~ — "• — *»-^ 

FIG.  297. — Transverse  section  of  a  small  root  of  the          FIG.  298. — Laticiferot 
andelion.  Scorzonera  (after  Sachs). 


FIG.  298. — Laticiferous  system 


not  infrequently  absent  in  the  stem  and  leaf,  but  appear  never  to 
be  lacking  in  the  root. 

Fig.  297  illustrates  the  organization  of  a  small  root  of  the 
common  dandelion  (Taraxacum).  The  phloem  of  the  fibro vascular 
system  is  much  better  developed  than  the  xylem,  and  in  it  occur 
certain  dark  concentric  bands.  These  indicate  the  position  of 
the  laticiferous  elements  which  elaborate  the  milky  juice,  so 
prominent  a  feature  of  the  Liguliflorae,  to  which  the  dandelion 


436  THE  ANATOMY  OF  WOODY  PLANTS 

belongs.  The  circular  zones  of  darker  elements  mentioned  above 
represent  not  only  the  distribution  of  the  laticiferous  tissue,  but 
that  of  the  sieve  tubes  as  well,  which  cannot  be  clearly  distinguished 
from  the  latter.  The  subject  of  the  relation  between  characteristic 
elements  producing  latex  and  sieve  tubes  needs  further  investiga- 
tion with  the  improved  methods  now  in  vogue.  Former  statements 
that  the  development  of  the  milk  system  is  in  inverse  proportion 
to  that  of  the  sieve  tubes,  which  have  been  both  denied  and 
affirmed,  may  advantageously  be  controlled  by  examination  of 
more  perfect  sections.  In  Fig.  298  is  seen  a  magnification  of  the 
laticiferous  system  of  the  black  salsify  (Scorzonera)..  It  is  here 
obvious  that  the  secretory  system  with  dark  contents  no  longer 
shows  the  identity  of  its  originally  separate  elements,  so  com- 
pletely has  cellular  fusion  taken  place. 

A  consideration  of  the  subfamily  Cynareae  of  the  Tubuliflorae 
will  next  occupy  our  attention.  Here  we  have  to  do  with  a  group 
which  is  transitional  anatomically  from  the  Tubuliflorae  to  the 
Liguliflorae,  since  it  possesses  partially  the  oil  canals  of  the  lower 
Compositae  and  likewise  the  laticiferous  system  which  is  a  feature 
of  organization  of  the  higher  members  of  this  important  group. 
Fig.  299  illustrates  a  portion  of  a  transverse  section  of  a  root  of 
the  so-called  French  artichoke,  Cynara  Scolymus.  Three  oil 
canals  are  to  be  seen  in  the  upper  region  of  the  section.  We  may 
now  pass  advantageously  to  a  consideration  of  the  common  bur- 
dock, Arctium  minor.  In  this  species  the  root  resembles  that  of 
the  artichoke,  figured  above,  in  the  possession  of  oil  canals.  The 
stem  of  the  burdock  in  its  lower  and  first-formed  region  presents 
some  interesting  features  of  organization,  which  are  illustrated 
in  Fig.  300,  showing  part  of  a  transverse  section  through  the 
axis.  In  the  lower  region  of  the  figure  appear  the  outer  parts  of  a 
number  of  fibrovascular  bundles  belonging  to  the  stem.  It  is 
clear  that  none  of  these  shows  the  presence  of  any  oil  canals. 
In  the  phloem  may  be  seen  dark  dots,  which  indicate  laticiferous 
elements.  Above  the  zone  of  stem  bundles  lies  a  single  leaf  trace 
in  the  cortex.  Along  the  outer  margin  of  this  appears  a  row  of 
secretory  canals.  The  nature  of  these  is  more  clearly  seen  in 
Fig.  301,  representing  the  transverse  view  of  the  foliar  trace  under 


EVOLUTIONARY  PRINCIPLES  OF  THE  COMPOSITAE      437 


FIG.  299. — Part  of  a  transverse  section  of  the  root  of  Cynara  Scolymus 


FIG.  300. — Transverse  section  of  the  axis  of  Arctium 


438  THE  ANATOMY  OF  WOODY  PLANTS 


FIG.  301. — Transverse  section  of  the  foliar  trace  of  Arctiwn 


FlG.  302. — Section  through  the  upper  aerial  stem  of  Arctium 


EVOLUTIONARY  PRINCIPLES  OF  THE  COMPOSITAE       439 

a  higher  degree  of  magnification.  External  to  the  fibrovascular 
tissues,  on  the  upper  side,  is  a  row  of  somewhat  imperfect  and 
vestigial  oil  canals.  The  interesting  fact  in  the  present  connection 
is  that  the  oil  canals  appear  in  relation  to  the  leaf  trace,  although 
absent  hi  the  ordinary  bundles  of  the  stem.  Sections  lower  down 
toward  the  hypocotyledonary  region  show  the  secretory  canals 
somewhat  better  developed.  In  no  case  do  they  occur  in  bundles 
which  belong  distinctly  to  the  stem,  although  they  may  run  a 
short  distance  down  on  the  foliar  traces  as  they  pass  into  the 
cylinder  of  the  axis.  In  the  upper  regions  of  the  stem  a  different 
situation  presents  itself.  Fig.  302  shows  a  section  through  the 
aerial  stem  of  the  burdock  in  transverse  section.  Here  neither 
the  numerous  bundles  of  the  axis  nor  any  of  the  three  foliar  strands 
show  the  presence  of  oil  canals,  although  small  dark  dots  in  the 
region  of  the  phloem  sufficiently  vouch  for  the  presence  of  laticifer- 
ous  elements.  Fig.  303  reproduces  a  magnified  view  of  one  of 
the  leaf  traces,  demonstrating  the  entire  absence  of  oil  canals 
in  the  foliar  strands  given  off  from  the  upper  part  of  the  axis. 
If  the  branches  are  followed  to  their  tips,  where  the  flowering 
burs  are  produced,  anatomical  investigation  reveals  the  continued 
absence  of  oil  canals. 

Similar  observations  may  readily  be  made  on  the  Scotch  thistle, 
Onopordon,  or  on  the  Canada  thistle,  Cirsium,  as  well  as  on  a 
number  of  other  representatives  of  the  Cynareae.  The  burdock 
is  on  the  whole  much  more  favorable  than  any  of  the  common 
thistles  and  has  accordingly  been  chosen  for  illustration.  It  will 
serve  a  useful  purpose  to  return  now  to  Cynara  Scolymus,  the 
French  artichoke.  As  is  well  known,  the  vegetable  so  designated 
is  a  variety  of  the  common  Mediterranean  thistle  in  which 
the  scales  of  the  involucre  surrounding  the  head  have  become 
hypertrophied  at  the  expense  of  the  floral  organs  proper.  This 
overgrown  involucral  covering  is  eaten  as  a  prized  vegetable  in 
Southern  Europe.  Fig.  304  shows  a  section  through  the  lower 
region  of  the  flowering  axis  of  Cynara  Scolymus.  The  bundles  on 
the  lower  side  of  the  figure  either  belong  to  the  axis  or  have  very 
recently  taken  their  departure  from  the  region  of  the  pith.  Those 
toward  the  upper  side  are  passing  into  the  basal  portion  of  the 


440 


THE  ANATOMY  OF  WOODY  PLANTS 


FIG.  303. — Highly  magnified  view  of  the  leaf  trace  in  Arctium 


FIG.  304. — Section  through  the  lower  region  of  the  flowering  axis  of  Cynara 
Scolymus. 


EVOLUTIONARY  PRINCIPLES  OF  THE  COMPOSITAE      441 

involucral  scales.  Certain  dark  dots  on  the  superior  margin  of 
the  upper  bundles  indicate  the  presence  of  oil  canals.  Structures 
of  this  nature  are  entirely  lacking  hi  the  axial  bundles  and  in  the 
traces  which  have  only  recently  left  the  cylinder  of  the  stem. 
Fig.  305  will  serve  to  demonstrate  the  truth  of  the  statement 
just  made.  In  a  is  shown  a  strand  near  the  axial  region  which 
obviously  is  not  characterized  by  the  presence  of  oil  canals.  In 
b,  on  the  contrary,  representing  a  trace  destined  to  an  involucral 


FIG.  305. — a,  axial  strand  of  infloresence  of  C.  Scolymus;  b,  a  strand  of  an  invo- 
lucral scale  in  the  same. 

scale,  a  series  of  somewhat  imperfectly  developed  oil  canals  appears 
along  the  upper  margin. 

The  Cynareae  represent  a  transition  anatomically  from  the 
Tubuliflorae  to  the  Ligulinorae.  In  the  Cichoriae,  on  the  other 
hand,  we  have  to  do  with  very  typical  representatives  of  the 
Liguliflorae,  since  in  this  subfamily  all  the  florets  are  strap-shaped 
and  the  secretion  is  milky  in  its  nature  and  contained  in  the  region 
of  the  phloem.  Interestingly  enough  in  the  oyster  plant  (Trago- 
pogon)  and  in  the  salsifies  (Scorzonera  and  Scolymus)  vestiges  of 


442  THE  ANATOMY  OF  WOODY  PLANTS 

an  oil-secreting  system  are  occasionally  found  in  the  root  only. 
Even  in  the  chicory  itself  some  slight  indications  of  oil  canals  are 
sometimes  discovered  in  the  roots,  although  these  are  never  func- 
tional as  in  the  salsifies. 

It  will  be  apparent  from  the  various  illustrations  and  state- 
ments of  the  foregoing  paragraphs  that  in  the  Compositae  the 
oil  canals  of  the  lower  forms  tend  to  perpetuate  themselves  in  a 
vestigial  fashion  in  certain  conservative  parts  and  organs  of  the 
higher  types  belonging  to  the  Cynareae  and  the  Cichorieae.  The 
general  situation  can  best  be  visualized  by  means  of  a  diagram 
which  is  made  in  accordance  with  the  data  supplied  by  the  French 
artichoke,  Cynara  Scolymus,  since  this  form  on  the  whole  reveals 
the  anatomical  situation  in  the  fullest  manner.  In  the  center 
of  Fig.  306  appears  a  somewhat  conventionalized  external  view 
of  the  species  under  discussion.  The  regions  which  are  of  critical 
importance  are  indicated  by  dotted  lines,  marked  by  letters  from 
A  to  D.  At  the  sides  are  shown  diagrammatic  transverse  sections 
of  the  regions  indicated  by  the  letters.  In  A  is  seen  a  view  of 
the  root,  conforming  to  the  general  structure  of  such  organs  and 
at  the  same  time  showing  a  series  of  oil  canals  just  external  to  an 
outer  circle  indicating  the  endodermis.  In  B  appears  a  view  of 
the  lower  region  of  the  axis  showing  leaf  traces,  either  present  in 
the  cylinder  of  the  stem  or  lying  externally  in  the  cortex.  In 
every  case  the  foliar  strand  is  accompanied  by  oil  canals,  which 
are  entirely  absent  in  the  bundles  of  the  stem.  In  C  is  represented 
a  section  through  the  high  aerial  portion  of  the  stem.  Here  the 
leaf  traces,  whether  still  within  the  cylinder  or  passing  outward 
in  the  cortex,  are  conspicuously  without  accompanying  oil  canals, 
as  are  also  those  of  the  axis  proper.  In  D,  which  illustrates  a 
region  of  section  through  the  base  of  the  reproductive  axis,  the 
oil  canals  appear  again  in  relation  to  the  traces  of  the  involucral 
appendages,  but  are  as  clearly  absent  in  the  axial  bundles  as  they 
are  in  the  lower  parts  of  the  stem.  It  is  accordingly  obvious  that 
the  various  canons  of  comparative  anatomy  stated  in  chapter  xvii 
are  as  well  illustrated  by  that  very  high  dicotyledonous  group, 
the  Compositae,  as  they  are  by  the  conifers  and  calamites.  More- 
over, while  it  is  true  that  in  the  case  of  the  high  dicotyledonous 


EVOLUTIONARY  PRINCIPLES  OF  THE  COMPOSITAE      443 

group  which  forms  the  subject  of  the  present  chapter  the  only 
data  for  regarding  the  Tubuliflorae  and  the  Liguliflorae  as  respec- 


FIG.  306. — Diagram  of  external  habit  and  distribution  of  oil  canals  in  the  arti- 
choke (C.  Scolymus). 

tively  lower  and  higher  are  morphological  in  their  nature,  the  lines 
of  evidence  in  this  connection  are  so  numerous  and  so  generally 
admitted  that  there  can  be  no  reasonable  doubt  as  to  their  validity. 


CHAPTER  XXXII 
ANATOMICAL  TECHNIQUE 

As  a  result  of  the  fact  that  it  has  been  mainly  the  soft  and  repro- 
ductive parts  of  plants  which  in  the  past  have  been  investigated 
in  connection  with  the  hypothesis  of  evolution,  the  methods  of 
examining  tissues  belonging  to  these  categories  have  naturally 
made  greater  progress  than  those  applicable  to  the  study  of  harder 
and  vegetative  structures.  The  reawakened  interest  in  fossil 
plants,  which  is  an  important  feature  of  the  present  phase  of  the 
development  of  morphology,  has,  however,  brought  into  promi- 
nence methods  for  the  anatomical  investigation  of  the  hard  tissues, 
since  in  general  it  is  only  the  harder  and  consequently  more  resist- 
ant parts  of  extinct  plants  which  furnish  a  basis  for  comparison 
with  modern  types.  This  general  situation  is,  moreover,  a  some- 
what fortunate  one,  for  it  is  precisely  the  enduring  woody  struc- 
tures which  combine  a  greater  immunity  to  decay  with  marked 
conservatism  of  organization.  Methods  of  investigating  hard 
tissues  in  plants  must  obviously,  from  the  conditions  outlined  above, 
include,  not  only  those  applicable  to  relatively  unaltered  living 
plants,  but  also  those  which  may  be  employed  with  success  in  the 
case  of  carbonized  or  petrified  vegetable  remains.  Since  the  pres- 
ent volume  deals  with  anatomy  alone,  it  will  be  possible  to  confine 
the  description  of  technique  to  those  processes  which  are  of  value 
in  the  investigation  of  the  hard  parts  of  plants. 

THE  PRESERVATION  OF  MATERIAL 

In  many  cases  the  preserving  of  material  of  hard  structures 
in  plants  does  not  present  a  problem  of  difficulty.  For  example, 
petrified  or  carbonized  stems  which  have  survived  the  destroying 
influences  of  often  extended  time  need  not  be  preserved  in  order 
to  be  studied  successfully.  In  such  cases  the  problem  of  the  prepa- 
ration of  the  tissues  rather  than  of  their  preservation  is  of  impor- 
tance. A  similar  situation  presents  itself  in  the  study  of  woods. 

444 


ANATOMICAL  TECHNIQUE  445 

Here  the  material  available  is  frequently  in  a  dried  condition;  and 
although  its  protoplasmic  structures  have  been  obliterated,  it  is 
still  by  reason  of  the  durability  of  its  essential  organization  emi- 
nently worthy  of  investigation.  Preservation  of  hard  structures, 
although  not  in  general  as  necessary  as  hi  the  case  of  the  softer 
tissues,  is  often,  however,  a  matter  of  prune  importance. 

The  skeletal  structures  of  plants  consist  of  more  or  less  thick- 
walled  cells  united  by  means  of  a  cement  substance  which  is  pectic 
in  its  nature.  The  process  of  preservation  must  be  such  as  will 
not  bring  about  maceration  of  the  hard  elements  by  the  dissolution 
of  the  bonding  material  or  middle  lamella.  Further,  preservatives 
which  increase  the  resistance  of  the  thick  cell  wall  to  sectioning 
are  in  general  undesirable.  As  a  result  of  the  conditions  just  indi- 
cated, chromic  acid  and  the  salts  of  chromium  cannot  in  general  be 
advantageously  employed  for  the  fixation  of  hard  tissues  in  plants, 
valuable  as  these  reagents  are  in  the  study  of  protoplasmic  struc- 
tures. Where  as  perfect  a  preservation  of  the  protoplasmic  organi- 
zation as  possible  is  essential,  and  at  the  same  time  the  avoidance 
of  undesirable  changes  in  the  thick  cell  wall  and  the  cement  sub- 
stance are  sought,  alcoholic  fluids  are  to  be  chosen.  Excellent  re- 
agents in  this  connection  are  solutions  of  corrosive  sublimate  or  picric 
acid  in  alcohol  of  from  100  to  30  per  cent  strength.  The  strength  of 
the  alcohol  will  depend  on  the  nature  of  the  material.  In  general, 
those  parts  which  by  reason  of  the  presence  of  a  large  proportion 
of  water  are  subject  to  shrinkage  in  strong  alcohol  should  be  fixed 
in  solutions  containing  a  lower  percentage,  while  harder  and  more 
impenetrable  tissues  may  be  treated  with  a  higher  grade  of  alcohol. 
In  many  instances  it  is  advisable  to  use  the  alcohol  without  any 
added  reagent.  This  is  quite  generally  the  case  with  wood;  for 
the  addition  of  corrosive  sublimate,  for  example,  is  not  of  sufficient 
advantage  in  connection  with  fixation  to  warrant  its  employment. 
Corrosive  sublimate,  picric  acid,  or  whatever  killing  reagent  may 
be  employed  in  conjunction  with  the  alcohol  can  conveniently  be 
used  in  saturated  solution  in  whatever  grade  of  alcohol  is  found  most 
advantageous.  Where  material  is  fixed  in  alcoholic  fluids  it  must 
be  washed  hi  alcohol  of  corresponding  or  greater  strength  to  remove 
the  excess  of  reagent .  Corrosive  sublimate  forms  a  black  precipitate 


446  THE  ANATOMY  OF  WOODY  PLANTS 

in  the  protoplasm  of  the  cells  which  is  to  be  removed,  after  the 
material  has  been  kept  in  strong  alcohol  for  some  time,  by  the  addi- 
tion of  crystals  of  metallic  iodine  until  the  alcohol  ceases  to  lose 
the  brown  color  imparted  by  the  iodine.  In  all  cases  the  preserved 
material  should  finally  be  brought  into  strong  alcohol  as  a  prelimi- 
nary to  further  treatment.  Where  the  parts  are  delicate,  such  as 
small  roots,  slender  herbaceous  stems,  or  the  organs  of  aquatics, 
alcohol  of  full  strength  should  be  reached  by  gradual  stages,  through 
30,  50,  70,  and  90  per  cent  grades. 

THE  MACERATION  OF  MATERIAL 

It  is  often  important,  particularly  in  the  case  of  the  more 
resistant  tissues,  to  separate  the  cells  from  one  another  by  the 
dissolution  of  the  cement  substance  or  middle  lamella.  This  end  is 
sometimes  attained  by  the  use  of  nitric  acid  and  a  chlorate.  Such  a 
procedure,  however,  is  unnecessarily  violent  and  causes  a  consid- 
erable amount  of  injury  to  the  cells  themselves.  A  better  method 
is  to  use  chromic  acid  in  conjunction  with  nitric  acid,  and  to  warm 
in  a  paraffin  bath,  if  it  is  desirable  to  hasten  the  process.  Macera- 
tion in  the  cold,  however,  often  gives  much  better  results.  The 
strength  of  chromic  acid  varies  according  to  the  material.  From 
5  to  10  per  cent  strength  of  the  two  acids  ordinarily  suffices  to 
bring  about  the  necessary  degree  of  maceration.  The  material 
after  soaking  for  some  time  in  the  macerating  fluid  may  be  washed 
in  repeated  changes  of  water  and  then  gently  scraped  with  the  point 
of  a  needle  or  scalpel.  As  a  result  of  this  procedure  masses  of  ele- 
ments may  be  removed  which  are  readily  teased  apart  with  needles 
or  separated  by  tapping  on  the  cover  glass. 

In  the  case  of  carbonized  material,  such  as,  for  example,  charred 
wood  in  coal,  commonly  but  not  very  aptly  designated  "mother 
of  coal,"  boiling  in  nitric  acid  often  yields  quantities  of  isolated 
tracheids  and  other  elements  of  the  wood  showing  all  the  details 
of  structure.  The  action  of  the  hot  nitric  acid  in  this  case  is  a 
double  one,  since  it  not  only  affects  the  isolation  of  the  elements, 
but  also  bleaches  the  black  hue  of  the  coal  to  a  light  brown.  In 
this  manner  details  of  structure  are  made  apparent.  Spores  are 
likewise  isolated  from  coal  by  this  method.  A  more  powerful  action 


ANATOMICAL  TECHNIQUE  447 

is  naturally  reached  by  the  use  of  aqua  regia  (nitro-hydrochloric 
acid)  or  of  nitric  acid  and  a  chlorate. 

THE   SOFTENING   OF   MATERIAL 

In  the  case  of  the  hard  tissues  of  plants  it  is  usually  necessary 
to  employ  some  method  of  softening  in  order  to  facilitate  the  sub- 
sequent cutting  of  sections.  It  has  been  discovered  that  the  hard- 
ness of  the  wall  of  the  cell  in  plants,  other  things  being  equal, 
depends  upon  the  amount  of  mineral  matter  present.  Lime, 
aluminum,  and  silicon  are  elements  often  found  in  the  wall,  par- 
ticularly where  it  is  considerably  thickened.  The  removal  of 
these  substances  depends,  of  course,  upon  the  use  of  an  appropriate 
acid,  and  hydrofluoric  acid  has  proved  most  useful  in  this  connec- 
tion. This  acid  has  the  advantage  of  not  attacking  the  middle 
lamella — an  advantage  which  is  of  considerable  importance  where 
good  sections  have  to  be  secured.  For  most  purposes  strong, 
commercial  hydrofluoric  acid  answers  sufficiently  well,  but  in 
cases  where  the  tissues  are  delicate  the  chemically  pure  reagent 
is  best  employed.  Hydrofluoric  acid  is  best  purchased  in  gallon 
leads  containing  about  ten  pounds  in  weight.  The  leads  should  be 
massive,  as  thin  ones  are  sometimes  eaten  through,  with  disastrous 
results,  before  the  acid  is  used  up.  The  material  is  prepared  for 
treatment  in  different  ways,  depending  on  its  nature.  In  the  case 
of  the  shell  of  a  hickory  nut  or  a  piece  of  ebony  or  live  oak,  the  dry 
material  is  first  boiled  for  some  time  in  water.  The  boiling  should 
be  continued  after  the  sinking  of  the  material  so  as  to  insure  the 
driving  out  of  all  air  and  the  complete  imbibition  of  the  mem- 
branes. It  is  then  allowed  to  cool  and  is  transferred  to  hydro- 
fluoric acid  of  full  strength.  After  sojourn  in  the  reagent  for  a 
week  or  two  a  piece  is  washed  for  a  short  time  in  running  water 
and  is  then  tested  with  a  knife  to  discover  if  it  cuts  readily.  If 
not,  a  longer  stay  in  acid  is  indicated,  and  in  case  of  very  hard  sub- 
stances the  time  of  sojourn  may  be  as  long  as  five  or  six  weeks  with 
occasional  renewal  of  the  reagent  in  extreme  instances.  The  hard- 
est and  most  refractory  tissues  of  existing  plants  may  be  mastered 
in  this  manner.  Wood  is  usually  cut  into  cubes,  which  may  advan- 
tageously be  about  one  centimeter  in  each  of  the  three  dimensions 


448  THE  ANATOMY  OF  WOODY  PLANTS 

and  should  have  their  faces  corresponding  with  the  transverse, 
radial,  and  tangential  planes  of  structure  of  the  wood. 

When  the  wood  is  softer,  after  the  preliminary  boiling,  which 
should  not  be  shortened,  the  pieces  of  larger  size,  on  account  of  the 
greater  ease  of  manipulation,  are  transferred  to  acid  which  has 
either  been  diluted  with  water  or  been  used  once  before  on  harder 
material.  In  this  they  are  left  for  a  shorter  time  than  is  necessary 
to  effect  the  softening  of  very  hard  tissues,  such  as  heavy  tropical 
woods  or  the  shells  of  nuts.  For  poplar  or  fir  wood  a  week  is  quite 
long  enough  to  bring  the  tissues  into  good  condition  for  sectioning. 

When  complete  organs  are  to  be  sectioned,  such  as  stems,  roots, 
leaves,  etc.,  a  preliminary  fixation  in  some  preserving  fluid  is 
necessary.  After  the  material  has  been  freed  of  the  fixing  agent, 
in  case  this  is  any  fluid  other  than  alcohol,  it  is  run  up  into  strong 
alcohol  and  then  freed  of  air  by  means  of  a  good  air  pump.  The 
air  must  be  removed  as  completely  as  possible  so  that  the  pieces 
will  sink  even  in  water.  After  the  pumping  has  been  completed, 
the  specimens  are  transferred  to  water  and  left  until  they  sink. 
If  the  parts  are  rather  delicate,  it  is  well  to  carry  on  the  process 
of  demineralization  in  weak  alcohol  instead  of  water  to  avoid 
maceration.  In  most  cases,  however,  if  the  material  has  been 
well  fixed  in  a  suitable  reagent,  no  appreciable  injury  is  caused 
either  to  the  cell  wall  or  to  the  protoplasm  by  the  use  of  hydro- 
fluoric acid. 

Naturally  the  demineralization  of  plant  tissues  of  whatever 
category  through  the  agency  of  hydrofluoric  acid  must  be  carried 
on  in  receptacles  made  of  wax  (either  hard  paraffin  or  beeswax 
will  serve) ;  or,  in  case  glass  bottles  are  used,  these  must  be  coated 
both  internally  and  externally  with  wax.  This  is  necessary  to 
avoid  the  destruction  of  the  container.  Gutta-percha  or  hard- 
rubber  bottles  are  sometimes  sold  for  the  purposes  indicated,  but 
they  are  not  so  resistant  to  acid  as  wax  and  are  unnecessarily 
expensive.  Further,  they  cannot,  like  wax,  be  used  with  chlorates 
in  certain  procedures  to  be  later  described.  Whatever  be  the 
nature  of  the  material  to  be  softened,  it  must,  after  remaining 
sufficiently  long  in  hydrofluoric  acid,  be  washed  entirely  free  of  this 
reagent  in  running  water.  The  washing  is  best  effected  in  the  case 


ANATOMICAL  TECHNIQUE  449 

of  small  pieces  in  a  vessel  covered  with  gauze  or  cheesecloth  to 
prevent  the  loss  of  material.  Larger  pieces  of  wood,  particularly 
when  they  are  heavy,  can  be  washed  in  open  bottles. 

The  next  stage  in  the  process  of  preparation  for  sectioning 
depends  on  the  nature  of  the  material.  If  the  objects  are  very 
small,  and  especially  if  they  lack  homogeneity  on  account  of  the 
presence  of  soft  as  well  as  hard  tissues,  they  must  be  subjected 
to  imbedding  in  nitrocellulose  or  paraffin,  and  the  former  process 
is  ordinarily  preferable.  In  the  case  of  larger  objects  one  of  two 
conditions  may  present  itself.  Either  the  structure  is  quite  homo- 
geneous, as  in  peachstones,  cubes  of  oak  wood,  etc.,  or  else  it  con- 
sists of  tissues  both  hard  and  soft,  as,  for  example,  segments  of 
stems  or  roots.  In  blocks  of  uniform  texture,  after  washing  is 
complete,  a  transfer  is  effected  to  a  fluid  consisting  of  equal  parts 
of  alcohol  of  30  per  cent  strength  and  glycerin.  Immersion  for 
some  days  in  this  reagent  fits  the  material  for  cutting.  When  the 
objects  are  of  some  size  and  are  not  of  homogeneous  organization 
they  are  run  up  through  alcohols  after  washing  to  a  strong  or  95 
per  cent  solution.  This  is  changed  twice  and  the  material  is  then 
placed  in  equal  parts  of  strong  alcohol  (95  per  cent)  and  glycerin. 
A  week  or  more  is  needed  under  these  circumstances  to  bring  about 
a  consistency  suitable  for  sectioning. 

IMBEDDING  IN  NITROCELLULOSE 

When  the  material  is  unhomogeneous  and  is  in  small  pieces, 
it  must  be  imbedded  in  nitrocellulose.  There  are  a  number  of 
different  types  of  nitrocelluloses  which  may  be  employed.  On 
the  whole,  the  best  for  the  purpose  is  Schering's  celloidin.  This 
is  a  special  preparation  of  particular  toughness,  solubility,  and 
purity.  When  extreme  transparency  is  a  desideratum,  the  nitro- 
body  known  as  photoxylin  may  be  used.  In  general,  however, 
a  nitrocellulose  which  is  entirely  transparent  is  quite  unnecessary, 
since  all  are  sufficiently  translucent  to  permit  the  ready  orienta- 
tion of  the  object  in  cutting.  The  cheapest  and  most  readily  pro- 
curable nitrocellulose  is  photographic  guncotton  prepared  for 
making  the  so-called  "wet-plates"  used  in  certain  photomechani- 
cal processes.  This  is  inexpensive  and  may  be  purchased  at  any 


450  THE  ANATOMY  OF  WOODY  PLANTS 

photographic  supply  store.  Whatever  nitrocellulose  is  used,  it 
must  be  first  washed  in  clean  water  and  then  carefully  dried.  The 
stock  should  always  be  kept  under  water  both  to  delay  changes  in 
chemical  composition  which  result  in  insolubility  and  to  avoid 
the  risk  of  explosion.  The  dry  nitrocellulose  is  dissolved  in  a 
mixture  of  equal  parts  of  good  ether  and  absolute  methyl  or  ethyl 
alcohol.  The  first  is  more  advantageous  because  it  forms  a  better 
solvent  of  the  nitro-body.  A  number  of  solutions  grading  from 
2  to  1 6  per  cent  must  ordinarily  be  prepared  and  kept  in  labeled 
bottles  with  wide  mouths  and  good  corks.  As  a  preliminary  to 
imbedding,  the  material  is  freed  of  all  gases  which  may  have  gath- 
ered in  its  intercellular  spaces  as  the  result  of  the  treatment  with 
hydrofluoric  acid  by  means  of  the  air  pump.  It  is  then  freed  of 
water  by  two  or  three  successive  changes  of  absolute  alcohol.  This 
reagent  is  best  purchased  in  gallon  bottles  from  a  reliable  manu- 
facturer, since  in  containers  of  one  pound  capacity  it  is  extremely 
expensive  in  the  United  States.  The  dehydrated  material  is  now 
ready  for  infiltration  with  nitrocellulose.  Small  strong  bottles  are 
prepared  by  twisting  tough  wire  about  their  necks  in  such  a  manner 
as  to  provide  two  short  and  diametrically  opposite  loops.  The 
wire  used  for  this  purpose  should  be  tenacious  and  not  too  slender. 
The  bottles  must  have  rather  wide  mouths  and  be  stopped  with 
corks  of  the  very  best  quality.  One  of  the  bottles  prepared  as 
described  is  partially  filled  with  nitrocellulose  of  the  2  per  cent 
strength,  and  the  objects  are  dropped  quickly  into  it  from  the 
absolute  alcohol  so  as  not  to  absorb  either  air  or  moisture.  There 
should  be  some  space  intervening  between  the  cork  and  the  solu- 
tion of  nitrocellulose  to  allow  for  expansion  in  heating.  The  cork 
is  finally  forced  in  and  a  piece  of  soft  tough  brass  or  copper  wire  is 
passed  through  the  loops  on  the  sides  of  the  neck  of  the  bottle  de- 
scribed above.  The  wire  is  then  drawn  tight  with  the  fingers  or  a  pair 
of  pliers  and  twisted  over  the  top  of  the  cork  so  as  to  be  held  securely 
in  place  even  when  the  bottle  is  exposed  to  the  heat  of  the  paraffin 
bath.  After  the  material  has  become  thoroughly  heated  the  con- 
tainers are  examined  for  the  purpose  of  detecting  any  leak  through 
the  cork.  Additional  security  against  escape  is  provided  by  keep- 
ing the  bottles  in  the  warm  bath  either  upside  down  or  on  their 


ANATOMICAL  TECHNIQUE  451 

sides.  It  is  usually  best  to  allow  the  material  to  remain  for  twenty- 
four  hours  in  the  first  grade  of  collodion  or  celloidin.  The  bottle  is 
then  cooled  in  the  air  or  more  quickly  by  cold  water.  The  fluid  is 
thrown  away  because  of  the  accumulation  of  extractives  from  the 
material.  Four  per  cent  of  celloidin  is  then  poured  on,  and  the 
bottle  wired  again  for  a  twelve-hour  immersion  in  the  warm  bath. 
The  second  solution  is  returned  after  cooling  to  its  proper  stock 
bottle,  and  the  successive  grades  of  dissolved  nitrocellulose  are  used 
respectively  until  16  per  cent  is  reached.  At  this  stage  chips  of 
nitrocellulose  previously  dissolved  and  then  dried  are  used  at  inter- 
vals and  in  a  small  amount  at  a  time  further  to  thicken  the  highest 
grade  of  solution.  The  bottle  is  returned  to  the  bath  with  each 
addition  of  dry  nitrocellulose.  If  the  process  has  been  carried 
on  with  proper  precautions,  the  material  has  becone  infiltrated 
without  shrinkage.  In  the  case  of  soft  material  which  is  at  the 
same  time  rather  impervious  it  is  often  an  advantage  thoroughly 
to  prick  the  pieces  with  a  very  fine  needle.  The  pricks  are  not 
usually  near  enough  to  injure  the  appearance  of  the  sections  which 
are  cut  later,  if  all  are  made  in  the  same  plane,  naturally  not  that 
of  greatest  importance  from  the  structural  standpoint. 

After  a  sufficient  degree  of  thickening  has  been  attained  in  the 
manner  described  in  the  foregoing  paragraph,  the  objects,  each 
surrounded  by  a  coating  of  nitrocellulose,  are  removed  to  chloro- 
form for  hardening.  The  chloroform  should  be  liberal  in  amount 
to  insure  thorough  induration  of  the  nitrocellulose,  and  the  mate- 
rial should  be  kept  in  the  reagent  for  at  least  twelve  hours.  Sub- 
sequently it  is  transferred  to  a  mixture  of  equal  parts  of  alcohol 
and  glycerin,  in  which  it  may  remain  until  needed  for  cutting. 
Objects  stored  in  this  way  seem  to  maintain  their  properties  indefi- 
nitely and  are  certainly  still  useful  after  as  long  an  interval  as  twenty 
years.  The  alcohol  should  be  prevented  from  evaporating  by 
occasional  renewals. 

SECTIONING   OF  MATERIAL 

Only  in  the  rarest  instances  is  it  desirable  to  cut  material  free- 
hand with  a  sectioning  razor.  If  uniformly  good  results  are  sought, 
the  microtome  should  be  used.  There  are  many  types  of  micro- 
tomes, but  those  with  the  simplest  mechanisms  and  the  greatest 


452  THE  ANATOMY  OF  WOODY  PLANTS 

degree  of  rigidity  are  most  useful  for  the  sectioning  of  hard  mate- 
rials of  vegetable  origin.  Microtomes  in  general  which  provide 
for  the  raising  of  the  object  by  a  vertical  screw  and  elaborate 
ratchet  devices  should  be  avoided,  as  they  are  very  difficult  to 
keep  in  order  when  the  sections  are  cut  wet  in  alcohol,  as  is  invari- 
ably the  case  with  the  materials  under  consideration.  A  slicing 
instrument  which  has  proved  itself  to  be  of  the  greatest  value  for 
the  purposes  here  enumerated  is  the  Thoma  microtome,  manufac- 
tured by  the  Jung  firm  of  Heidelberg.  The  so-called  Naples 
model  is  the  best  size  and  type  and  has  the  advantage  of  being 
immune  from  rust  by  reason  of  its  phosphor-bronze  construction. 
In  this  microtome  the  object  is  raised  by  sliding  up  an  inclined 
plane  through  the  action  of  a  micrometer  screw  which  is  accurately 
gauged  by  means  of  an  adjustable  clicking  ratchet  device.  The 
horizontal  position  of  this  mechanism  and  its  manipulation  by 
hand  are  great  advantages,  since  by  reason  of  these  features  it 
escapes  the  corrosion  which  rapidly  impairs  vertical  screws  for 
regulating  the  thickness  of  the  sections.  For  cutting  small  objects 
the  carrier  provided  with  the  instrument  which  can  be  orientated 
in  two  planes  is  suitable.  The  object-carrier  is  provided  with 
small  cylinders  which  are  primarily  intended  for  use  with  paraffin. 
The  hollow  of  the  cylinder,  instead  of  being  filled  with  paraffin  as 
in  its  ordinary  mode  of  employment,  is  blocked  by  a  well-seasoned 
and  accurately  rounded  piece  of  wood.  This  should  be  dicoty- 
ledonous and  rather  coarse-grained.  The  smooth  upper  surface  of 
the  block  is  varnished  thoroughly  (it  is  generally  best  to  dip  the 
end  of  the  block  into  the  solution)  with  a  4  per  cent  solution  of 
nitrocellulose.  If  the  blocks  are  being  used  for  the  first  time,  they 
should  receive  a  second  coat  of  celloidin  (or  collodion)  after  the 
first  has  had  time  to  dry.  The  objects  imbedded  in  celloidin  (or 
collodion)  are  now  removed  and  examined  with  a  lens  to  ascertain 
the  plane  in  which  they  are  to  be  sectioned.  When  decision  is 
reached  on  this  point,  the  surface  of  the  hardened  nitrocellulose 
is  sliced  with  a  sharp  knife  in  a  plane  parallel  to  that  of  the  desired 
sections.  The  smoothed  surface  should  be  perfectly  flat.  After 
wiping,  this  surface,  which  should  never  actually  expose  the  object, 
is  clipped  into  a  4  or  6  per  cent  solution  of  nitrocellulose  and  then 


ANATOMICAL  TECHNIQUE  453 

firmly  pressed  for  a  moment  on  the  prepared  surface  of  one  of  the 
wood-blocked  cylinders.  After  resting  for  ten  or  fifteen  minutes 
in  a  warm  place  it  becomes  securely  fixed  and  is  ready  for  cutting. 
The  knives  for  cutting  hard  vegetable  materials  should  not  be 
too  thinly  ground  on  the  edges.  The  type  of  edge  supplied  by 
the  Jung  firm  under  the  designation  of  c  answers  very  well  for  the 
purpose.  The  knives  are  best  not  too  large  in  size  and  should  not 
exceed  eight  inches  in  length.  They  are  sharpened  by  the  aid  of  a 
cylindrical  appliance  slipped  over  the  back,  which  gives  the  edge 
a  less  acute  angle  to  the  sharpening  stone,  with  a  corresponding 
advantage  in  saving  of  time.  The  stone  or  hone  for  putting  the 
edge  of  the  knife  in  condition  should  be  preferably  a  yellow  Belgian 
one,  such  as  is  ordinarily  employed  by  barbers,  but  of  considerably 
larger  size  and  of  as  fine  texture  as  can  be  procured.  It  is  better, 
in  fact,  to  have  two  stones,  one  coarser  for  preliminary  use  and  a 
finer  one  for  finishing.  Carborundum  hones,  although  often  sup- 
plied for  the  purpose  of  sharpening  microtome  knives,  are  not 
advantageous.  In  renewing  the  edge  of  the  knife  care  should  at 
first  be  taken  to  remove  all  hacks  or  gaps  by  grinding  on  a  coarse 
hone.  In  the  sharpening  process  the  edge  of  the  knife  should  be 
pushed  forward  on  the  stone  and  not  backward,  as  the  latter  pro- 
cedure results  in  a  so-called  "wire  edge."  After  the  removal  of 
the  nicks  the  edge  is  finished  on  a  finer  hone.  If,  as  a  result  of 
frequent  sharpening,  the  knife  has  been  ground  away  to  a  very 
thick  edge  and  consequently  sharpens  very  slowly,  it  is  necessary 
to  grind  it  on  an  alundum  or  carborundum  wheel  of  very  fine  tex- 
ture. The  knife  is  held  on  a  support  known  as  a  knife-grinder  and 
brought  against  the  revolving  wheel  at  such  an  angle  that  the  edge 
is  slightly  hollow-ground.  It  requires  a  little  skill  to  grind  a  knife 
on  the  wheel,  but  this  is  quickly  acquired  and  the  frequent  sending 
of  the  knives  to  the  cutler  is  thus  avoided.  A  further  advantage  of 
grinding  the  knives  in  the  laboratory  is  the  avoidance  of  drawing 
of  the  temper,  which  is  often  the  unfortunate  result  of  sending 
them  to  a  professional  grinder.  The  wheel  should  be  revolved  at 
a  high  rate  of  speed  and  the  edge  of  the  knife  held  gently  against 
it.  In  this  manner  overheating  is  avoided.  The  grinding  machines 
furnished  by  the  Carborundum  Company  of  Niagara  Falls  and  the 


454  THE  ANATOMY  OF  WOODY  PLANTS 

Luther  Company  of  Milwaukee  are  excellent,  particularly  the 
latter. 

After  the  edges  have  been  sharpened  on  the  grinding  wheel 
and  hone  the  knives  are  finished  by  means  of  a  strop.  The  best 
type  has  four  sides  and  a  solid  wooden  center.  The  four  sides  are 
covered  with  leather  surfaced  in  various  manners.  The  first  face 
provides  a  coarse  polish  and  the  two  following  ones  successively 
finer  degrees.  The  last  face  is  of  smooth  leather  and  imparts  the 
final  smoothness  to  the  edge.  The  knife  should  be  carefully  wiped 
with  a  cloth  after  being  passed  on  the  successive  surfaces  to  avoid 
the  impairment  of  the  finer  ones  by  material  from  the  coarser 
grades.  In  stropping,  the  edge  should  be  drawn  backward  and 
not  pushed  forward  as  in  honing,  since  failure  to  observe  this  pre- 
caution results  in  injury  to  the  leather  surfaces.  Strops  consisting 
of  unsupported  leather  are  not  desirable  for  use  with  microtome 
knives. 

The  knives  are  held  in  place  on  the  microtome  by  the  knife- 
holder,  which  in  turn  is  fastened  to  a  heavy  block  of  phosphor- 
bronze  running  in  a  channel  on  the  right  side  of  the  microtome. 
The  holder  may  be  altered  in  position  on  the  block,  forward  and 
backward,  inward  and  outward,  by  means  of  appropriate  screw 
holes.  It  may  be  raised  by  plates  provided  for  insertion  beneath 
its  shaft.  The  best  type  of  holder  is  provided  with  a  tilting  mech- 
anism which  makes  it  possible  to  vary  the  horizontal  angle  of  the 
knife  to  the  object.  In  cutting  all  hard  tissues  but  coal  the  micro- 
tome knife  should  be  in  an  oblique  position  to  the  object,  and  in 
general  the  inclination  should  be  somewhat  acute.  In  the  case  of 
large  objects  and  of  those  which  are  likely  to  curl  in  cutting,  a 
more  oblique  position  is  sometimes  advantageous.  In  making 
sections  the  surface  of  the  knife  should  be  kept  wet  with  alcohol 
of  95  per  cent  strength.  This  is  ordinarily  effected  by  means  of  a 
camel's-hair  brush.  This  is  also  used  for  preventing  the  curling 
of  the  section  as  it  comes  up  on  the  knife.  The  proper  touch  for 
flattening  the  section  without  either  rolling  or  dragging  it  is  acquired 
by  experience  only.  Sections  may  be  cut  from  two  or  three  micra 
to  many  times  that  thickness,  depending  on  the  particular  condi- 
tions involved.  If  the  sections  curl  after  being  placed  in  alcohol 


ANATOMICAL  TECHNIQUE  455 

subsequent  to  removal  from  the  edge  of  the  knife,  this  disadvantage 
may  be  largely  obviated  by  allowing  them  nearly  to  dry  on  the 
flat  surface  of  the  knife  at  some  distance  from  the  part  of  the  edge 
which  is  actually  being  used  for  cutting. 

In  the  case  of  hard  and  homogeneous  material,  such  as  wood, 
pieces  of  nutshell,  coal,  etc.,  a  special  type  of  object-holder  must 
be  used,  in  which  the  material  is  firmly  held  in  a  rigid  clamp.  Two 
varieties  of  these  have  been  devised  for  the  Thoma  microtome, 
recommended  for  the  purposes  here  described.  One,  contrived 
by  Professor  R.  B.  Thomson,  is  made  in  Toronto,  and  particulars 
in  regard  to  it  may  be  obtained  by  writing  to  the  Botanical  Depart- 
ment of  the  University  of  Toronto.  This  is  an  admirable  device 
and  is  particularly  useful  in  cutting  large  pieces  of  wood  which 
have  not  been  sufficiently  softened.  A  second  type  of  holder  for 
hard  objects  has  been  devised  by  the  writer  and  is  manufactured 
to  order  by  the  Jung  Company  of  Heidelberg.  In  these  holders 
the  position  of  the  object  can  be  varied  so  as  to  obtain  the  proper 
inclination  to  the  edge  of  the  knife.  For  example,  in  cutting  radial 
sections  of  wood  the  slices  obviously  must  be  accurately  parallel 
to  the  rays,  or  else  a  very  confused  condition  is  presented  under 
the  microscope.  The  object-carriers  in  both  these  holders  are 
very  heavy  in  order  to  insure  the  necessary  rigidity  and  inertia. 
A  complete  Thoma  microtome  with  the  additional  holder  for 
cutting  hard  objects  costs  about  one  hundred  dollars  free  of 
American  duty.  With  the  Thomson  devices  the  cost  is  consider- 
ably greater. 

As  has  been  pointed  out  in  an  earlier  paragraph,  wood  and 
similar  tissues  need  not  be  infiltrated  with  nitrocellulose  to  secure 
the  best  results.  If,  however,  the  woody  tissues  for  any  reason 
have  become  unduly  softened,  either  from  too  prolonged  immer- 
sion in  hydrofluoric  acid  or  through  the  ravages  of  fungi  (this  is, 
of  course,  particularly  the  case  where  diseased  or  rotten  wood  is 
being  studied),  imbedding  in  celloidin  or  collodion  is  necessary. 
As  will  be  shown  in  a  later  paragraph,  this  procedure  is  absolutely 
necessary  with  most  coals.  The  cutting  of  sections  of  yery  hard 
tissues  involves  the  same  principles  as  exemplified  in  the  technique 
of  smaller  and  less  homogeneous  objects,  with  the  exception  only 


156  THE  ANATOMY  OF  WOODY  PLANTS 

3f  coal,  which  demands  a  transversely  placed  instead  of  an  oblique 
knife.  In  cutting  more  resistant  plant  substances  the  edge  of  the 
knife  must  be  very  smooth  and  sharp  and  must  be  much  more 
irequently  renewed  than  in  the  case  of  softer  materials. 

It  is  also  possible  in  certain  cases  to  section  unhomogeneous 
materials,  consisting  of  pieces  of  root,  stem,  etc.,  by  adopting  the 
iollowing  procedure:  When  the  sections  are  transverse,  the  pieces 
nay  be  clamped  directly  in  the  jaws  of  the  wood-holder  described 
ibove.  In  the  case  of  longitudinal  sections,  either  tangential  or 
radial,  the  objects  are  removed  from  the  mixture  of  equal  parts 
)f  strong  alcohol  and  glycerin  and  accurately  smoothed  on  one 
surface  by  means  of  a  sharp  knife.  The  plane  surface  is  dried 
:arefully  with  a  cloth  and  then  painted  with  a  6  per  cent  solution 
}f  celloidin.  After  the  film  of  nitrocellulose  has  thoroughly  dried 
:he  object  is  quickly  dipped  in  4  per  cent  solution  and  applied  to 
:he  surface  of  a  block  of  wood  which  has  previously  been  varnished 
ivith  nitrocellulose  in  the  manner  described  in  a  former  paragraph. 
By  clamping  the  block  in  the  wood-holder  it  is  possible  to  secure 
extremely  thin  longitudinal  sections  without  the  labor  involved 
n  imbedding  in  nitrocellulose.  It  should  be  emphasized,  how- 
ever, that  this  method  is  not  available  in  the  case  of  organs  which 
show  a  considerable  diversity  of  texture.  For  example,  the 
branches  of  Ginkgo  and  Tsuga  must  be  imbedded  to  secure  success- 
ful sections,  while  those  of  Pinus  and  Quercus  provide  good  trans- 
verse preparations  without  previous  infiltration. 

Other  vegetable  materials  can  best  be  prepared  as  thin  sections 
without  any  previous  treatment  whatever.  This  is  true  of  peri- 
ierm,  as  illustrated  by  common  bottle  cork  and  "birch  bark." 
Phese  tissues  need  merely  to  be  clamped  in  the  microtome  to  fur- 
nish sections  as  good  as  may  be  secured  by  the  most  elaborate 
processes  of  softening  and  imbedding.  Fresh  leaves  also  provide 
very  thin  sections  when  treated  in  the  following  manner:  Rather 
thick  leaves  are  best  in  the  case  of  the  dicotyledons  and  mono- 
cotyledons, for  example,  in  Rhododendron  and  Yucca.  These  are 
beld  between  two  smooth  pieces  of  pine  wood  in  the  jaws  of  the 
wood-holder.  Sections  are  made  until  the  knife  has  cut  down 
sufficiently  near  to  the  pieces  of  pine  to  secure  rigidity  in  the  leaf 


ANATOMICAL  TECHNIQUE  457 

tissues.  The  sections  are  then  cut  thin  by  appropriate  manipu- 
lation of  the  micrometer  screw  and  floated  instantaneously  on  the 
edge  of  the  knife  in  alcohol.  Thence  they  are  transferred  to  a  dish 
of  water  and  show  all  their  natural  organization  and  color  prac- 
tically unchanged.  Sections  as  thin  as  five  and  ten  micra  are 
easily  obtained  in  this  manner  and  are  very  striking.  The  leaves 
of  gymnosperms  such  as  cycads  and  conifers  lend  themselves  to 
the  same  treatment. 

METHODS    OF    STAINING 

The  thin  sections  prepared  by  the  methods  above  indicated 
are  frequently  so  tenuous  as  to  present  insufficient  detail  on  micro- 
scopic examination.  This  inconvenience  is  ordinarily  overcome 
by  the  use  of  stains  to  bring  out  contrasts  in  structure.  The  sub- 
ject of  staining  has  been  so  recently  and  admirably  discussed  for 
American  students  in  Professor  Chamberlain's  Methods  in  Plant 
Histology  (3d  ed.,  The  University  of  Chicago  Press,  1915)  that  it 
need  only  be  elucidated  in  the  present  connection  in  regard  to  the 
special  conditions  presented  by  the  hard  tissues  in  plants. 

A  few  stains  give  the  best  results  in  most  anatomical  investi- 
gations, since  the  protoplasmic  structures  are  of  less  importance 
and  the  cellulosic  and  lignified  conditions  of  the  cell  wall  are  of  the 
greatest  significance.  In  most  instances  staining  with  hematoxy- 
lin  and  counterstaining  with  the  anilin  dye  known  as  safranin  give 
the  best  results.  In  the  case  of  hematoxylin  the  Haidenhain 
method  on  the  whole  answers  best.  The  sections  are  washed  in 
distilled  water  and  then  immersed  in  a  3  per  cent  solution  of 
ammonia-iron  alum.  The  alum  should  be  bluish  in  color  and  free 
from  efflorescence.  Ten  to  fifteen  minutes  in  the  alum  solution 
are  sufficient.  Careful  washing  in  distilled  water  follows.  The 
first  change  of  washing  water  should  remain  only  a  short  time  and 
is  followed  by  a  second  and  a  third  change,  so  as  to  remove  all 
alum  which  has  not  become  fixed  in  the  sections.  In  material 
containing  a  great  deal  of  tannin  special  precautions  must  be  taken 
to  insure  thorough  washing  by  repeated  changes  of  distilled  water. 
When  the  sections  are  cleared  of  excess  of  alum,  they  are  ready 
for  the  hematoxylin  solution,  which  in  some  cases  is  employed  in 


458  THE  ANATOMY  OF  WOODY  PLANTS 

the  strength  of  one-half  of  i  per  cent  in  distilled  water.  In  some 
instances  this  is  too  great  a  concentration  and  leads  to  overstaining. 
This  may  be  remedied  by  the  transfer  of  the  sections  to  distilled 
water  with  a  few  drops  of  iron-alum  solution  in  which  bleaching 
slowly  takes  place.  When  a  sufficient  degree  of  decoloration  has 
been  attained,  the  sections  must  be  washed  in  repeated  changes 
of  distilled  water  as  in  the  first  procedure,  otherwise  the  blue  colora- 
tion will  fade  in  a  short  time.  A  better  method  is  to  stain  more 
gradually,  using  only  a  few  drops  of  the  solution  of  hematoxylin 
described  above.  Slow  staining  in  general  gives  better  results 
than  rapid.  The  blue-tinted  sections  when  properly  colored  are 
transferred  to  distilled  water  and  thence  to  distilled  water  to  which 
a  drop  or  two  of  safranin  has  been  added.  This  stain  is  prepared 
by  adding  equal  parts  of  water-soluble  and  alcohol-soluble  safranin 
to  strong  alcohol  until  a  saturated  solution  has  been  reached. 
The  coloration  with  safranin  is  best  carried  on  slowly  and  in  dilute 
solution,  as  in  this  way  both  clear  detail  and  strong  contrast  are 
best  secured.  Very  often  the  best  results  follow  from  leaving  the 
sections  overnight  in  the  dilute  safranin.  In  this  instance,  how- 
ever, precaution  should  be  taken  to  wash  the  sections  very  care- 
fully and  repeatedly  after  the  use  of  iron  alum  and  hematoxylin, 
as  otherwise  disagreeable  precipitates  are  likely  to  make  their 
appearance. 

Some  plant  tissues  and  remains  show  to  best  advantage 
without  staining.  This  is,  for  example,  frequently  true  of  darkly 
colored  heartwood  and  of  fossil  plants  and  coal.  In  the  case  of 
the  two  latter  it  is  often  expedient  to  reduce  the  color  by  bleaching 
agents.  The  most  convenient  method  in  this  connection  is  sup- 
plied by  the  use  of  chlorine  water.  Hydrogen  peroxide,  so  often 
recommended  for  bleaching  vegetable  tissues,  is  of  no  value  in  the 
case  of  carbonized  and  fossil  plant  remains.  When  the  dark  colora- 
tion is  very  intense,  a  more  vigorous  bleaching  action  results  from 
the  use  of  nascent  chlorine,  which  can  be  conveniently  secured  by 
dissolving  a  few  crystals  of  a  chlorate  in  distilled  water,  with  the 
subsequent  addition  of  a  small  quantity  of  nitric  acid.  This  com- 
bination gives  sufficiently  vigorous  bleaching  action  without  accom- 
panying maceration. 


ANATOMICAL  TECHNIQUE  459 

METHODS   OF  MOUNTING 

In  the  case  of  the  great  mass  of  vegetable  tissues  it  is  frequently 
an  advantage  to  bring  about  the  sharpest  possible  accentuation 
of  the  color  contrasts  resulting  from  staining  by  mounting  in  media 
of  high  refractive  index.  For  this  purpose  the  sections  must  be 
usually  entirely  freed  from  water.  This  end  is  effected  by  means 
of  absolute  alcohol.  The  sections  are  transferred  on  a  section- 
lifter  from  the  solution  of  safranin  to  a  watch-glass  of  absolute 
alcohol,  excess  of  fluid  having  been  removed  by  touching  the  lifter 
to  a  piece  of  blotter  or  filter  paper.  From  the  first  absolute 
alcohol  they  are  lifted  to  a  second,  which  effects  the  final  removal 
of  water.  Too  long  immersion  in  the  first  absolute  alcohol  is 
likely  to  extract  too  much  of  the  safranin  stain,  but  a  longer  stay 
is  advantageous  if  the  sections  have  received  an  excessive  colora- 
tion in  red.  In  case  the  sections  have  been  cut  from  material 
infiltrated  with  nitrocellulose,  the  latter  may  often  be  retained 
with  advantage.  This  object  is  attained  by  adding  a  little  chlo- 
roform to  the  two  changes  of  absolute  alcohol,  this  procedure  hav- 
ing the  effect  of  preventing  the  softening  of  the  nitrocellulose. 
Should  it  appear  desirable  to  remove  the  infiltrating  substance 
completely,  the  sections  are  transferred  to  ether  after  the  second 
absolute  alcohol.  In  any  case,  after  dehydration  is  effected 
in  any  of  the  ways  described,  the  sections  are  finally  cleared 
with  a  reagent  of  high  refractive  index.  The  most  convenient 
of  these  for  vegetable  preparations  is  chemically  pure  and  an- 
hydrous benzene  or  benzole.  After  clarification  the  preparations 
are  ready  for  mounting.  This  is  effected  in  hard  Canada  balsam, 
dissolved  in  whatever  agent  has  been  used  to  clear  the  sec- 
tions. Benzole,  xylol,  chloroform,  etc.,  are  employed  for  this 
purpose. 

The  solution  of  balsam  should  not  be  too  dilute,  as  the  evapora- 
tion of  the  solvent  may  under  this  condition  result  in  the  disas- 
trous desiccation  of  the  sections.  Even  where  the  section  is  not 
actually  left  bare  by  the  drying  out  of  the  mounting  balsam,  the 
access  of  air  is  likely  to  promote  a  rapid  fading  of  the  colors.  It  is 
usually  necessary  to  flatten  the  sections  because  the  balsam  sets 
by  the  evaporation  of  the  solvent.  This  procedure  is  essential 


460  THE  ANATOMY  OF  WOODY  PLANTS 

in  cases  where  the  structures  present  are  to  be  reproduced  by  pho- 
tographic methods.  After  the  preparations  have  dried  for  a  day 
or  two  in  a  horizontal  position  at  laboratory  temperature,  they  are 
weighted  with  lead  in  the  form  of  rolls.  For  photomicrographic 
purposes  the  final  degree  of  flattening  is  secured  by  the  pressure 
of  clip  clothespins  which  are  prevented  from  injuring  the  thin 
covers  by  the  interposition  of  thin  slices  of  cork.  In  the  processes 
of  flattening  described  above,  the  preparations  are  subjected  to 
continually  increasing  heat.  This  is  conveniently  secured  by 
laving  the  slides  upon  a  board  on  top  of  a  steam  radiator.  The 
end  of  the  board  nearer  the  supply  pipe  of  the  radiator  is  naturally 
much  hotter  than  that  nearer  the  return.  The  final  flattening  and 
setting  of  the  sections  is  best  carried  on  in  the  interior  of  the  par- 
affin bath,  which  is  an  adjunct  of  every  botanical  laboratory. 
It  is  unnecessary,  of  course,  to  resort  to  such  extreme  measures 
to  secure  flatness  unless  the  preparation  is  to  be  reproduced  by 
photomicrographic  methods. 

In  a  number  of  instances,  especially  when  the  sculpture  of  the 
cell  wall  in  elements  of  the  wood  is  a  particular  feature  of  interest, 
mounting  in  Canada  balsam  brings  about  too  great  a  degree  of 
homogeneity.  This  inconvenience  can  be  avoided  by  using  other 
media  for  mounting.  Gum  dammar  dissolved  in  chloroform  sup- 
plies a  mountant  which  presents  many  of  the  advantages  of  Canada 
balsam  without  the  extreme  degree  of  clarification  which  makes 
that  reagent  in  some  cases  undesirable.  Glycerin  jelly  is  an  even 
better  mounting  medium,  but  has  the  disadvantage  of  tending  to 
extract  the  safranin.  This  can  be  largely  overcome,  however,  by 
transferring  the  sections  after  staining  to  glycerin  containing  a 
large  amount  of  safranin.  Under  these  circumstances  the  safranin 
is  not  extracted  from  the  sections.  In  mounting,  the  sections  are 
placed  upon  the  slide  and  warmed  over  a  burner  to  make  the  gly- 
cerin quite  fluid.  This  is  then  allowed  to  drain  off  as  much  as  pos- 
sible, and  any  remaining  trace  is  removed  by  wiping  round  the 
section  with  a  piece  of  clean  cotton  fabric.  A  drop  of  melted  gly- 
cerin jelly  is  then  applied  and  the  cover  is  placed  in  position. 
Weighting  is  advisable  during  setting  and  cooling  of  the  jelly  to 
secure  flattening  of  the  sections. 


ANATOMICAL  TECHNIQUE  461 

METHODS   OF   SECTIONING  COAL 

This  mineral  is  now  universally  recognized  to  be  of  vegetable 
origin.  Its  practical  importance  and  the  interesting  remains 
which  it  contains  make  it  an  object  not  without  interest  both  from 
the  general  botanical  and  from  the  anatomical  standpoints.  All 
except  the  most  highly  carbonized  coals  may  be  prepared  for 
sectioning  on  the  microtome  without  great  difficulty  in  accord- 
ance with  the  summary  method  here  described.  The  mineral  is 
split  in  conformity  to  the  layers  by  means  of  a  stout  knife  and  a 
hammer.  The  thin  slabs  thus  secured  are  broken  transversely 
by  the  aid  of  pliers  or  a  chisel  into  pieces  which  vary  in  size  accord- 
ing to  the  resistance  of  the  coal.  The  fragments  are  put  into 
melted  phenol  or  carbolic  acid  and  kept  in  corked  bottles  in  the 
warm  bath  for  about  a  week.  They  are  then  transferred  after 
washing  in  warm  water  to  strong  hydrofluoric  acid  for  a  second 
week.  Except  in  the  case  of  certain  lignitic  coals  further  treat- 
ment with  phenol  after  washing  with  water  is  necessary.  This 
is  followed  by  a  second  immersion  in  hydrofluoric  acid  and  subse- 
quent washing.  After  a  third  return  to  phenol  (if  this  is  necessary) 
the  material  is  washed  in  water  and  then  run  up  into  absolute 
alcohol  which  must  be  changed  two  or  three  times  During  the 
process  of  dehydration  the  coal  is  kept  in  or  on  the  paraffin  bath. 
Imbedding  in  nitrocellulose  follows.  In  this  process  only  2,  4 
and  6  per  cent  solutions  of  celloidin  are  used,  and  the  last  of  these 
is  followed  at  once  by  nitrocellulose  of  16  per  cent  strength.  It. 
is  an  advantage  in  imbedding  coal,  not  only  to  pump  all  air  out  of 
its  substance,  but  also,  when  in  the  6  per  cent  celloidin,  to  subject 
to  a  positive  pressure  of  between  two  and  three  hundred  pounds. 
This  is  easily  effected  by  inclosing  in  a  metal  cylinder  and  raising 
the  internal  pressure  by  means  of  an  automobile  pump.  After  a 
rapid  course  of  thickening  rendered  possible  by  the  resistant  char- 
acter of  the  material,  the  pieces  are  dropped  into  chloroform  in  the 
usual  manner  and  later  transferred  to  glycerin  and  strong  alcohol. 
In  sectioning,  the  fragments  of  coal  are  clamped  securely  in  the 
wood-carrier  of  the  microtome  and  cut  with  a  transversely  placed 
knife.  Only  sections  which  are  five  micra  or  less  in  thickness  are 
of  value  in  the  case  of  this  extremely  opaque  material.  The 


462  THE  ANATOMY  OF  WOODY  PLANTS 

utilization  of  botanical  technique  in  the  study  of  coal  has  appar- 
ently made  it  necessary  to  revise  the  accepted  views  as  to  the  origin 
of  this  invaluable  mineral.  The  hypothesis  of  the  derivation  of 
coals  from  peat  formed  in  place,  as  in  the  northern  bogs  of  our 
epoch,  must  be  given  up  in  favor  of  the  conception  of  an  accumu- 
lation in  open  water  by  transport  either  aerial  or  aquatic.  Coal 
from  its  internal  organization  obviously  is  comparable  to  the  muck 
in  the  bottoms  of  modern  lakes  rather  than  to  the  surface  accumu- 
lations of  peat  which  often  surround  them.  The  technique  of 
mounting  sections  of  coal  is  the  same  as  for  imbedded  material  in 
which  the  matrix  of  nitrocellulose  is  retained.  No  staining  is 
necessary. 

PHOTOMICROGilAPHIC   METHODS 

Photography  with  the  microscope  has  become  in  recent  years 
an  extremely  important  aid  in  microscopical  investigations  of  the 
structure  of  plants.  To  such  perfection  have  photographic  lenses 
and  photographic  plates  attained  that  it  is  possible  to  reproduce 
photographically  most  of  the  structural  features  of  tissues  and  cells 
in  plants.  The  advantage  of  photomicrographic  reproduction  is 
that  it  is  at  once  less  laborious  and  more  accurate  than  represen- 
tation by  drawings. 

The  lenses  used  for  photomicrographic  purposes  do  not  differ, 
except  in  special  cases,  from  those  employed  in  general  microscopic 
investigation.  For  very  low  magnifications  such  as  are  ordinarily 
^not  available  with  the  compound  microscope  special  lenses  of  great 
perfection  are  now  manufactured  by  the  leading  makers  of  micro- 
scopical appliances.  These  are  used  without  eyepieces  and  can 
be  applied  as  a  rule  only  to  special  photomicrographic  stands. 
For  moderate  and  higher  magnifications,  however,  the  same  optical 
outfit  is  used  as  for  observation  with  the  eye.  The  better  grades  of 
lenses  naturally  give  better  photographic  results  than  those  which 
are  of  inferior  quality.  In  the  case  of  very  high  magnifications 
special  condensers,  insuring  a  great  degree  of  concentration  of 
light  and  freedom  from  chromatic  aberration,  are  employed  fcr 
illumination. 

Microscopic  lenses  have  certain  general  defects  which  militate 
against  their  employment  for  photographic  purposes.  The  most 


ANATOMICAL  TECHNIQUE  463 

serious  of  these  is  the  discrepancy  between  the  chemical  and  visual 
focus.  As  a  result  of  this  shortcoming,  a  picture  sharply  focused 
to  the  eye  appears  dull  and  indistinct  upon  the  photographic  plate. 
In  the  case  of  apochromatic  objectives  and  compensation  oculars 
this  optical  defect  is  less  apparent  than  in  ordinary  so-called  achro- 
matic lenses  and  Huygenian  eyepieces.  The  lack  of  correspond- 
ence of  visual  and  chemical  foci  becomes  practically  negligible 
where  certain  color  screens  are  used  in  securing  the  photographic 
image.  In  some  instances,  particularly  in  the  photography  of 
unstained  material,  lenses  of  rock  crystal  are  of  value,  since  they 
shut  out  less  actinic  light  than  do  lenses  of  glass.  The  great  cost, 
however,  of  lenses  of  this  construction  renders  them  unavailable 
for  most  laboratories,  and  in  any  case  their  value  is  by  no  means 
proportionate  to  their  price. 

Since  microscopic  sections  are  generally  colored  either  natu- 
rally or  by  means  of  special  stains,  photographic  plates  which  are 
sensitive  to  color  are  necessary  in  photomicrography.  There  are 
various  types  of  such  plates  which  are  available  for  different  pur- 
poses. It  has  been  found  that  certain  chemical  substances,  par- 
ticularly anilin  dyes,  possess  the  valuable  property  of  greatly 
increasing  the  sensitiveness  of  the  photographic  plate  to  colors. 
The  photographic  negative  is  ordinarily  produced  on  a  gelatin 
surface  mounted  on  glass.  The  gelatin  covering  is  known  as  the 
emulsion  and  contains  bromide  of  silver  precipitated  in  its  sub- 
stance. The  silver  bromide  is  rendered  more  or  less  sensitive  to 
light  by  boiling  in  the  presence  of  ammonia,  on  the  one  hand,  or 
by  adding  an  excess  of  bromide  of  potash,  on  the  other.  It  is  most 
sensitive  to  the  radiations  of  the  violet  and  ultra-violet  region  of 
the  spectrum.  Consequently  the  common  photographic  plate  will 
not  give  good  results  in  the  case  of  sections  stained  with  red,  green, 
or  even  very  dark  blue  dyes.  This  original  defect  of  the  photo- 
graphic plate  has  been  almost  entirely  removed  in  recent  years  by 
the  use  of  anilin  sensitizers.  For  example,  erythrosin  possesses 
the  property  of  rendering  plates  soaked  in  a  weak  solution  sensi- 
tive to  greens.  Plates  which  are  so  treated  are  ordinarily  known 
as  isochromatic.  The  appellation  is,  however,  obviously  a  mis- 
nomer, since  they  are  only  in  a  very  slight  degree,  if  at  all,  sensitive 


464  THE  ANATOMY  OF  WOODY  PLANTS 

to  reds.  The  most  valuable  type  of  plate  for  general  photomicro- 
graphic  purposes  is  that  treated  with  a  blue  anilin  dye,  isocyanin. 
This  treatment  makes  the  emulsion  sensitive  to  the  red  end  of  the 
spectrum  to  a  very  large  degree.  Such  plates  are  called  "pan- 
chromatic" or  "spectrum"  plates  and  are  of  the  greatest  value 
in  the  photomicrographic  reproduction  of  extremely  difficult  micro- 
scopic objects,  such  as  coal,  highly  tanniferous  stems,  roots,  etc. 

Further  aids  to  the  microscope  in  the  objective  reproduction  of 
microscopic  images  are  the  so-called  color  screens.  These  are  of 
various  hues,  but  need  not  be  considered  in  detail,  since  only  a  few 
of  them  are  practically  useful  for  photomicrographic  purposes. 
When  a  yellow  screen  is  employed,  for  example,  it  shuts  off  to  a 
large  extent  the  violet  and  ultra-violet  radiations  and  as  a  conse- 
quence gives  the  photographic  plate  greater  efficiency  in  the  repro- 
duction of  colors  other  than  light  blue.  A  further  extremely 
valuable  result  following  the  use  of  a  yellow  screen  is  the  elimina- 
tion of  the  contrast  between  chemical  and  visual  foci  mentioned 
above  as  a  disadvantageous  feature  distinguishing  microscopic 
lenses  from  those  employed  in  ordinary  photographic  cameras. 
This  is  a  property  of  the  utmost  practical  importance  and  makes 
available  lenses  which  would  otherwise  be  quite  useless.  The 
general  effect  of  a  yellow  screen,  then,  is  to  make  a  plate  relatively 
more  sensitive  to  radiations  other  than  the  violet  ones  and  at  the 
same  time  to  favor  a  more  brilliant  microscopic  image  as  a  result 
of  the  elimination  of  the  ordinary  chemical  or  actinic  rays.  A 
green  screen  is  in  certain  cases  highly  useful,  but  the  numerous 
other  hues  which  distinguish  screens  made  for  the  appropriate 
rendering  of  color  are  in  general  of  less  value  in  the  practice  of 
photomicrography. 

The  photomicrographic  apparatus  may  advantageously  con- 
sist of  an  ordinary  camera  with  long  bellows,  supported  in  a  ver- 
tical position  on  a  counterpoised  sliding  back.  The  back  may  be 
fixed  at  any  height  by  means  of  window  fasteners.  The  camera 
is  connected  with  the  microscope  by  a  collar  which  engages  with 
a  corresponding  collar  on  the  tube  of  the  microscope  in  such  a  man- 
ner as  to  break  the  path  of  the  rays  of  light.  Reflections  are  pre- 
vented by  the  blackening  of  the  collar  rings  with  matt  drop  black. 


ANATOMICAL  TECHNIQUE  465 

A  shutter  which  fits  on  the  front  of  the  photographic  camera  is 
most  convenient,  and  an  arrangement  which  will  admit  of  exposures 
of  from  three  seconds  to  a  hundredth  part  of  a  second  is  desirable. 
The  Thornton-Pickard  shutter,  of  English  manufacture,  but  used 
a  great  deal  in  the  United  States,  has  proved  itself  of  value  in  this 
connection.  When  a  color  screen  is  used,  it  may  be  laid  horizon- 
tally just  above  the  shutter  in  view  of  the  vertical  position  of  the 
camera.  A  special  photomicrographic  microscope  is  desirable, 
although  not  absolutely  necessary,  as  any  ordinary  microscope  of 
good  size  and  weight  with  adequate  optical  equipment  may  be 
employed  for  photomicrographic  work.  The  microscope  is  sup- 
ported on  a  bench  provided  with  three  legs  so  that  it  may  be  always 
steady.  The  upright  stand  to  which  the  camera  is  attached  should 
likewise  have  three  brass  knobs  screwed  into  its  base  to  insure 
stability.  The  best  source  of  light  is  an  arc  lamp  with  hand  feed. 
The  ordinary  projection  lantern  answers  very  well"  for  this  pur- 
pose if  it  is  not  too  cumbersome.  A  condenser  should  be  placed 
in  front  of  the  arc  to  collect  the  light,  and  a  water  chamber  should 
likewise  be  interposed  between  the  condenser  and  the  microscope 
to  eliminate  heat  rays.  The  use  of  alum  in  the  water  chamber  to 
accentuate  the  exclusion  of  the  heat  radiations  is  unnecessary,  and 
it  is  inadvisable  on  account  of  the  evil  effects  produced  on  the 
apparatus  by  alum.  The  lantern  should  be  so  mounted  that  it 
can  readily  be  lowered  and  raised,  and  also  inclined  at  any  angle. 
A  fixed  lantern  may  be  employed,  but  its  utility  is  much  restricted. 
The  carbons  are  best  inclined  at  an  angle  of  ninety  degrees.  The 
light  is  taken  from  the  end  of  the  positive  carbon,  which  should  be 
in  a  horizontal  position.  The  current  should  be  direct  and  not 
alternating,  although  the  latter  may  be  used  with  some  degree  of 
success.  The  advantage  of  the  direct  current  is  the  freedom  from 
noise  and  the  intense  light  emanating  from  the  positive  carbon. 
The  amperage  should  be  controlled  by  a  rheostat  which  will  per- 
mit the  passage  of  from  seven  or  eight  to  fifteen  amperes  of  current. 
The  high  amperages  are  used  in  the  case  of  dark-colored  or  opti- 
cally opaque  objects.  In  the  case  of  photographs  under  low  magni- 
fication a  ground-glass  screen  may  often  be  inserted  with  advantage 
between  the  mirror  of  the  microscope  and  the  water  chamber. 


466  THE  ANATOMY  OF  WOODY  PLANTS 

The  photographic  dark  room  should  be  of  moderate  size  and 
provided  if  possible  with  electric  lights.  In  addition  to  a  free 
light  there  may  be  one  shaded  by  a  ground-glass  cover  for  examin- 
ing negatives  and  exposing  lantern  slides.  Two  safe  lights  for 
developing  are  likewise  necessary.  One  of  these  should  be  screened 
with  orange  and  deep-ruby  glasses,  one  behind  the  other.  This 
light  is  available  for  the  development  of  the  less  sensitive  chro- 
matic plates,  known  as  iso-plates,  and  for  all  ordinary  photo- 
graphic plates.  A  second  safe  light  is  necessary  for  use  with  plates 
sensitized  by  isocyanin  (so-called  spectrum  and  panchromatic 
plates),  which  must  ordinarily  be  developed  in  total  darkness. 
The  Wratten  &  Wainwright  Company,  of  Croyden,  England,  has 
patented  a  light  which  may  be  used  for  this  purpose,  and  this  is 
on  sale  in  the  better  photographic  supply  houses  in  the  larger 
American  cities.  It  is  indispensable  for  all  highly  color-sensitive 
plates.  A  sink  of  good  length  and  with  two  taps  should  be  provided. 
To  the  left  there  should  be  a  gently  sloping  draining-board  with 
shelves  beneath  and  above  for  developing-  and  fixing-trays  of 
hard  rubber,  as  well  as  developers  and  glassware.  Ventilation  of 
the  dark  room  is  desirable,  and  is  absolutely  essential  in  warm 
weather.  It  is  best  effected  by  means  of  an  ordinary  electric  fan 
placed  near  the  door  of  the  room  in  such  a  position  as  to  cause 
strong  currents  into  and  out  of  the  dark  room.  It  may,  of  course,  be 
set  in  a  special  light-tight  chamber  communicating  with  the  out- 
side, but  this  fixed  position  considerably  restricts  its  usefulness. 

The  plates  employed  for  making  negatives  depend  on  the  par- 
ticular needs  in  special  cases.  For  very  exacting  work  in  which 
the  object  is  either  extremely  dark-colored  or  opaque  the  use  of 
panchromatic  or  spectrum  plates  is  indicated.  In  case  the  con- 
trasts are  very  slight,  and  particularly  in  weakly  stained  material 
or  objects  which  present  only  a  slight  natural  coloration,  the  so- 
called  iso-process  plate  is  valuable.  For  the  mass  of  colored  prepa- 
rations the  ordinary  isochromatic  plate  answers  every  purpose. 
A  common  need  in  connection  with  teaching  is  the  copying  of 
illustrations.  When  these  consist  of  line  or  half-tone  engravings, 
process  plates  giving  a  high  degree  of  contrast  are  employed.  It 
was  formerly  necessary  to  import  color-sensitive  plates  from 


ANATOMICAL  TECHNIQUE  46 

Europe,  but  these  are  now  made  in  good  variety  and  of  excellen 
quality  by  the  Cramer  Dry  Plate  Company,  of  St.  Louis,  Missour 
All  the  types  of  plates  described  above  may  be  purchased  from  th 
Cramer  Company.  Since  ordinary  photographic  supply  house 
often  do  not  carry  in  stock  the  special  plates  needed  for  photc 
micrographic  and  other  scientific  purposes,  the  Cramer  Compan 
undertakes  to  furnish  these  direct  from  its  factory  in  St.  Louis. 

The  making  of  lantern  slides  is  an  activity  which  may  often  b 
pursued  profitably  in  larger  botanical  establishments  in  univei 
sities.  The  most  convenient  method  of  making  such  slides  is  b 
placing  the  lantern  plate  directly  behind  a  negative  of  suitabl 
size,  namely,  3!  by  4^  inches.  A  few  seconds'  exposure  to  a  ligr 
sheltered  by  ground  glass  in  the  dark  room  (the  time  dependin 
on  the  character  of  the  negative)  suffices.  Development  of  lai 
tern  slides  is  effected  by  means  of  a  special  developer  to  be  mei 
tioned  in  a  subsequent  paragraph.  The  slides  must  be  covere 
with  a  mask  which  can  be  made  by  cutting  out  black  paper  b 
means  of  a  wheel  paper-cutter  and  appropriate  metal  forms,  obtaii 
able  at  photographic  supply  stores.  Masks  may  also  be  purchase 
with  the  various  forms  and  sizes  of  opening  necessitated  by  tl 
different  kinds  of  pictures.  After  the  mask  is  applied  the  lantei 
slide  is  protected  by  a  thin  cover-glass  held  in  position  by  blac 
paper  binding  strips  which  may  be  procured  in  any  photograph 
establishment.  American  and  Continental  European  lantern  slidi 
are  3!  by  4  inches  in  dimension.  The  English  slide  is  3^  inchi 
square.  The  storing  of  lantern  slides  is  always  a  problem  whe 
their  number  becomes  large,  and  many  filing  cabinets  for  this  pu 
pose  are  on  the  market.  The  essential  thing,  however,  is  to  ha~\ 
numbers  on  the  slides  and  a  catalogue,  so  that  they  can  readi] 
be  selected  for  use  and  easily  returned  to  their  places.  The  con 
plexity  of  the  catalogue  will  of  course  depend  upon  the  tastes  an 
needs  of  the  individual  and  the  extent  of  his  budget. 

The  development  of  the  various  plates  described  in  the  for 
going  paragraph  is  effected  by  reagents  which  reduce  to  metall 
silver  the  parts  of  the  plate  exposed  to  the  light.  Developers  a; 
legion,  but  two  or  three  seem  to  answer  every  practical  purpos 
For  making  negatives  pyrogallol  developer  is  on  the  whole  mo 


468  THE  ANATOMY  OF  WOODY  PLANTS 

advantageous.  This  is  made  by  dissolving  one  ounce  of  Mallinck- 
rodt  or  Schering  pyrogallol  in  150  cubic  centimeters  of  tap  water, 
acidulated  beforehand  with  20  drops  of  nitric  acid.  This  solution 
should  be  kept  in  the  dark  room  and  is  known  as  "stock  pyro." 
A  second  solution  is  made  by  adding  to  tap  water  10  per  cent  of 
carbonate  of  soda  and  10  per  cent  of  sulphite  of  soda,  together 
with  a  quarter  of  i  per  cent  of  bromide  of  potash.  In  this  solution 
the  carbonate  of  soda  is  used  (with  the  pyrogallol)  for  the  purpose 
of  reducing  the  silver  in  the  exposed  plate.  The  sulphite  of  soda 
prevents  staining,  and  the  bromide  of  potash  restrains  a  too  rapid 
development  and  insures  a  clear  image.  To  make  the  develop- 
ing solution,  dilute  one  volume  of  the  "stock  pyro"  with  nine 
parts  of  water.  This  is  solution  No.  I.  Solution  No.  II  is  the  one 
described  above  containing  carbonate,  sulphite,  and  bromide. 
Of  No.  I  and  No.  II  take  equal  parts.  The  developer  should  be 
poured  quickly  and  evenly  over  the  plate  and  air  bubbles  should 
be  avoided.  The  time  of  development  depends  upon  the  nature 
of  the  plate,  but  ordinarily  the  image  should  begin  to  appear  in 
from  thirty  to  sixty  seconds.  Development  is  continued  until 
the  image  begins  to  disappear.  Lantern  slides  and  bromide  prints 
are  best  developed  with  metol,  which  is  an  organic  developer  of 
German  origin  used  almost  exclusively  in  the  manufacture  of 
moving-picture  films.  Its  employment  in  this  connection  is  suffi- 
cient proof  of  its  value.  The  developer  is  made  by  adding  together 
in  the  proportion  of  one  to  three  the  solutions  described  below 
under  the  denominations  A  and  B.  Solution  A  is  made  by  adding 
10  per  cent  of  caustic  soda  or  caustic  potash  to  distilled  water. 
Solution  B  consists  of  10  per  cent  sulphite  of  soda,  i  per  cent  metol 
(Metol-Hauff),  and  a  quarter  of  i  per  cent  bromide  of  potash  in 
distilled  water.  After  the  two  solutions  are  mixed  they  may  be 
diluted  with  advantage  by  adding  a  third-  to  a  half-volume  of 
tap  water.  Development  of  lantern  plates  and  bromide  paper  is 
ordinarily  complete  in  from  fifty  to  sixty  seconds.  The  lantern 
plates  are  best  exposed  in  the  dark  room,  while  it  is  often  advan- 
tageous from  the  standpoint  of  time-saving  to  expose  printing 
papers  to  daylight.  Velox  papers,  and  in  particular  Glossy  Velox 
(ordinary  or  special  depending  on  the  vigor  of  the  negative),  may 


ANATOMICAL  TECHNIQUE  469 

be  used  for  making  prints  for  reproduction  in  scientific  journals. 
The  smooth,  glossy  surface  renders  this  type  of  paper  more  suit- 
able for  purposes  of  reproduction  than  the  more  artistic  papers 
with  matt  or  rough  surfaces. 

Photomicrography  is  an  art  which  can  be  learned  only  in  part 
from  books,  and  the  beginner  is  advised  either  to  visit  some  lab- 
oratory where  it  is  successfully  carried  on,  or,  failing  that,  to  seek 
direction  from  a  professional  photographer,  having  regard  in  that 
case  to  the  special  conditions  involved  in  photomicrographic  tech- 
nique. The  account  given  above  is  from  limitation  of  space  neces- 
sarily incomplete  and  has  been  written  only  to  supply  some  answer 
to  numerous  inquiries  as  to  the  methods  pursued  in  the  laboratories 
of  plant  morphology  in  Harvard  University. 


INDEX 


INDEX 


Abies,  root  of,  135,  149,  334 

Abieteae,  334 

Abietineae,  325;  antiquity  of,  325;  bars 
of  Sanio  in,  326;  cone  axis  of,  328;  re- 
lation of,  to  Ginkgoales,  337;  relation 
of,  to  other  Coniferales,  334;  resin 
canals  of,  334;  wood  of,  326 

Adiantum,  stem  of,  279 

Agathis:  leaf  trace  of,  324;  seedling  of, 
323;  wood  of,  319;  A.  BidwiLlii,  330, 
353 

Air  spaces,  3 

Alburnum  or  sap  wood,  55,  56,  57 

Alcohol,  445 

Alnus:  leaf  of,  212;  rays  in,  153,  177 

Amphivasal  bundles,  distribution  of,  in 
monocotyledons,  414 

Anatomical  technique,  444 

Angiosperms:  general  characters  of,  373; 
reproductive  structures  of,  374;  vessels 
of,  95;  wood  of,  373 

Annual  rings,  15,  16,  421 

Annulus  in  Pteridophyta,  216 

Araucaria  Bidwillii,  323 

Araucariineae,  318;  alternating  pitting 
in,  319;  bars  of  Sanio  in,  323;  per- 
sistent leaf  trace  of,  324;  wood  of,  318 

Araucariopitys,  353 

Araucarioxylon,  349;  pitting  of,  320; 
structure  of,  320 

Archaeocalamitaceae,  266 

Archigymnospermae,  292,  305 

Arctium,  distribution  of  oil  canals  in,  437 

Arrangement  of  bundles:  in  Equise tales, 
273;  in  monocotyledons,  192 

Aster:,  oil  canals  of,  434;  resemblance  of 
stem  to  that  of  the  oak,  406;  root  of, 
434;  stem  of,  405 

Astromyelon,  276 

Avena,  bundle  of,  411 

B 

Barberry,  substitute  fibers  of,  34 
Bars  of  Sanio,  68;  diagnostic  value  of,  in 
Coniferales,  323 


Bast  fibers,  118 

Bast,  hard  and  soft,  118 

Bennettitales,  303;  bisporangiate  cone  of, 

303;  course  of  leaf  traces  in,  301 
Betula,  wood  rays  of,  155 
Bleaching,  446 

Bocoa  provacensis,  tracheids  of,  33 
Brachyoxylon,    wood    of,    320;     wound 

reactions  of,  321,  329 
Bundle  in  monocotyledons,  194,  195 


Calamitaceae,  251;  course  of  bundles  in, 

273 
Catamites,    266;     pith    casts    of,    269; 

primary  wood  of,  266,  268;   secondary 

wood  in,  268;  young  stem  in,  266 
Callus:   definitive,  123;   in  phloem,  no, 

114;  seasonal,  123 
Cambial  activity  in  monocotyledons,  195, 

411 

Camera,  photomicrographic,  464 
Canada  balsam,  459 
Carbonized  material,  bleaching  of,  446 
Casuarina:   leafy  twig  of,  211;    rays  of, 

77,  82,  404;    C.  equisetifolia,  rays  of, 

78,  88,  90;   C.  Fraseri,  rays  of,  77,  86; 
C.   torulosa,  rays  of,   77;    systematic 
position  of,  384;   transfusion  tissue  of, 
384 

Cedrus,  resin  canals  of,  74,  335 
Cedroxylon,  structure  of,  349 
Cell,  i-s 
Celloidin,  449 

Centripetal  wood  in  Equisetum,  274 
Chalazogamy  in  Amentiferae,  230 
Chamaecyparis,  marginal  tracheids  of,  72 
Chicory,  degenerate  oil  canals  in  root  of, 

442 

Chlorates,  446,  458 
Chlorine,  458 
Chloroplastids,  2 
Chromic  acid,  445,  446 
Cirsium,  oil  canals  of,  439 
Clematis,  wood  rays  of,  179,  191 


473 


474 


THE  ANATOMY  OF  WOODY  PLANTS 


Climate  in  relation  to  evolution  of  plants, 
4i7,  423 

Coal,  sectioning  of,  461 

Collateral  protostele,  165 

Collateral  siphonostele,  165 

Collodion,  449 

Color  screens,  464 

Color-sensitive  plates,  463 

Companion  cells  in  angiosperms,  118 

Comparative  anatomy,  canons  of,  234 

Coinpositae:  illustrative  of  general  prin- 
ciples of  anatomy,  433;  organization  of 
stem  in,  400,  402,  404 

Compound  ray,  origin  of,  82,  181 

Concentric  bundles  in  monocotyledons, 
i95 

Concentric  protostele,  162 

Concentric  siphonostele,  163 

Coniferales,  317;  genealogical  tree  of, 
355;  subtribes  of,  355 

Conifers,  phylogeny  of,  355 

Conservative  organs,  238;  leaf,  238;  re- 
productive axis,  238;  root,  240 

Cordai tales,  305;  leaf  in,  203,  309;  rays 
of,  66;  stem  in,  306;  transfusion  tissue 
of,  208,  308;  transition  region  in,  309; 
wood  of,  15,  28,  66,  305,  308,  419 

Corrosive  sublimate,  445 

Cortex,  10 

CrystaHogenous  cells,  118 

Cupressineae,  339;  marginal  tracheids  of, 
34i 

Cupressinoxylon,  349 

Cuticle,  128 

Cycadales,  297;  centripetal  wood  of  leaf, 
300,  of  cone  axis,  299;  supernumerary 
zones  in,  298 

Cycadofilicales,  292 

Cycas:  cortical  bundles  in,  201;  leaf 
bundle  of,  199;  leaflet  of,  200 

Cynara  Scolymus,  distribution  of  oil 
canals  in,  436,  439,  443 

Cystoliths,  127 


Dammar,  460 

Dandelion,  root  of,  435 

Depressed  segments:    in  Aster,  405;    in 

Clematis,  179;  in  Quercus,  180 
Developing  reagents,  468 
Diagram  of  distribution  of  oil  canals  in 

artichoke,  443 


Dicotyledons:  characteristics  of,  375; 
degenerate  vessels  of,  373,  380;  modifi- 
cations of  tracheids  in,  381;  parenchym- 
atous  elements  of,  381;  phylogeny  of, 
384;  wood  rays  of,  382 

Diffuse  ray,  origin  of,  82,  89 

Distribution  of  amphivasal  bundles  in 
monocotyledons,  412 

Duramen  or  heartwood,  56,  57,  58 


Ectokinetic  sporangium,  216 

Electrical  current  for  photomicrography, 

465 

Endarch,  23,  167 
Endodermis,  10,  n,  12 
Endokinetic  sporangium,  217 
Ephedra,  357;  rays  of,  174,  358;  stem  of, 

i?3,  358;  tracheids  of,  31;  wood  of,  94 
Epidermis,    126;    multiple,    127;    water 

tissue,  127 
Equisetaceae,  269;   course  of  bundles  in, 

273 

Equisetales,  250,  264;  stem  of,  272 
Equisetum,  269;  bundles  of,  270;  cone  of , 

274;  leaf  trace  of,  274;  medulla  of,  2 72; 

primary  wood  of,  270;    root  of,  275; 

stem  of,  269;  types  of  stem  in,  271 
Erianthus,  bundle  of,  195 
Ersatzfasern,  34 
Exarch,  23,  167 

F 

Fagus,  tracheids  of,  32 
Ferns,  277;  root  of,  150;  stem  of,  278 
Fiber  tracheids,  31;  tylosis  of,  106 
Fibers:  libriform  origin  of,  32;  mucilagi- 
nous, 33;  septate,  34;  substitute,  34 
Filicales,  277;  leaf  of ,  280;  stem  of,  278; 

root  of,  20,  150 
Fixing  reagents,  445 
Fundamental  tissues,  132 
Fusiform  rays:  in  Cedrus,  74;   in  Pinus, 

etc.,  69 

G 

Geinitzia,  355 
Ginkgo:   absence  of  parenchyma  in,  424; 

alternate  pitting  in,  312;  leaf  of,  313; 

opposite   pitting  in,   311;    secondary 

wood  of,  311;    sporangia  and  sporo- 

phylls  of,  314;    tangential  pitting  of, 

423 
Ginkgoales,  310;    relation  of,  to  Abie- 

tineae,  337 


INDEX 


475 


Gleichenia,  stem  of,  278 

Gnetales,  357;    and  Bennettitales,  370; 

parenchyma  in,  358;    vessels  in,  358; 

vine  habit  in,  364 

Gnetum,  363;  stem  of,  363;  vessels  of ,36  7 
Grape  vine,  septate  fibers  of,  34 
Grinding  wheels,  453 
Guard  cells,  129;  number  of,  131 

H 

Hairs,  130 

Hard  tissues,  preparation  of,  for  sec- 
tioning, 447 

Heartwood:  condition  of  parenchyma 
in,  56,  57;  heartwood  or  duramen,  55 

Helianthus:  compared  with  Casuarina, 
404;  diagram  to  illustrate  herbaceous 
type,  403;  H.  hirsutus,  ray  situation  in, 
400,  402,  404;  H.  tuberosus,  oil  canals 
of,  434 

Hematoxylin,  457 

Herbaceous  and  arboreal  types  in  rela- 
tion to  climatic  conditions,  430 

Herbaceous  dicotyledons,  186 

Herbaceous  type:  geographical  distribu- 
tion of,  430;  in  geological  tune,  431; 
origin  of,  in  cryptogams,  387,  in  the 
dicotyledons,  186,  387,  in  Helianthus, 
400,  402,  404,  in  Potentilla,  187,  in 
Salvia,  189,  in  Urtka,  397 

Herbs,  substitute  fibers  of,  35 

Heterangium,  stem  of,  293 

Hones,  types  of,  453 

Hydrofluoric  acid,  uses  of,  447 

Hypoderma,  128 


Imbedding  in  nitrocellulose,  450 

Inclusion  of  pith:  within  leaf  traces,  285; 
within  stele  of  stem,  284 

Intercellular  spaces,  2 

Iodine,  uses  of,  446 

Iris,  pericarp  of,  2 

Iron  alum,  457 

Isoetes,  255 

K 

Kinds  of  plates  useful  for  photomicrog- 
raphy, 463,  466 

Knives,  453;  position  of,  453 

L 

Labiates,  origin  of  herbaceous  type  in,  189 
Lamium,  origin  of  herbaceous  type  in,  190 


Lantern  slides,  467 

Larch,  root  of,  22,  145 

Large  storage  rays  in  relation  to  climatic 

conditions,  430 
Laticiferous  system  in  the  Compositae, 

435 

Lattices,  122 

Leaf:  centripetal  wood  of,  in  Prepinus, 
208;  concentric  bundles  of,  in  Cycas, 
201,  302;  cortical  bundles  of,  in  Cycas, 
201,  302;  of  Cycadales,  199,  300;  defi- 
nition of,  141 ;  gap  of,  in  Casuarina, 
81,  in  conifers,  80,  in  Cordaitales,  203; 
organization  of,  in  Alnus,  213,  in 
Casuarina,  211,  in  dicotyledons,  213, 
in  Gnetales,  211,  in  monocotyledons, 
213,  in  Finns,  205,  330,  in  Prepinus, 
207.  331;  rachis  of  Cycadales,  199; 
transfusion  tissue  in  Casuarina,  311, 
in  Cordaitales,  203,  in  Pinus,  206,  in 
Prepinus,  207;  vegetative,  199 

Leea,  organization  of  stem  in,  184,  389, 
396;  origin  of  woody  cylinder  of,  184, 
389 

Leguminosae:  fibers  and  tracheids  of,  33; 
origin  of  herbaceous  type  in,  191 

Lenses,  photomicrographic,  462 

Lepidocarpon,  224 

Lepidodendraceae,  250,  255;  secondary 
wood  of,  1 6,  260 

Lepidodendron,  256;  primary  wood  of, 
170;  rays  in,  258;  septate  tracheids  of, 
38 

Libriform  fibers,  32 

Lights  for  photomicrographic  purposes, 
466 

Ligulifloreae,  laticiferous  system  of,  435 

Liquidambar,  fibers  of,  106 

Liriodendron,  parenchyma  of,  51;  vessels 
of,  97,  zoo 

Lycopodiales,  250,  252 

Lycopodium,  19,  252;  root  and  stem  of, 
143 

Lycopods,  wood  of,  252 

Lycopsida,  geological  range  of,  244; 
microphylly  in,  245 

Lyginodendron,  296 

M 

Maceration,  446 

Magnolia,  fiber  tracheids  of,  32;  paren- 
chyma of,  51 

Marginal  tracheids:  of  Cupressineae,  73, 
341;  of  Taxodineae,  72,  339 


476 


THE  ANATOMY  OF  WOODY  PLANTS 


Medulla:  cortical  origin  of,  283;  in  Os- 
mundaceae,  286;  origin  of,  in  the 
Filicales,  284,  in  Lycopsida,  283 

Medullary  rays:  so  called  in  gymno- 
sperms,  172;  in  Lycopsida,  170 

Medullosa,  294;  origin  of  cycads  from, 
295 

Megasporangium:  in  Pteridophyta,  223; 
in  Selaginella,  216 

Mesarch,  23,  167 

Mesophyll,  213 

Metagymnospermae,  317,  357 

Metaxylem,  19 

Methods  of  sectioning  coal,  461 

Miadesmia,  225 

Microscope  in  photoniicrography,  465 

Microscopic  lenses,  defects  of,  462 

Microsporangium:  in  Conifers,  219;  in 
Cycadales,  216;  in  Ginkgoales,  218; 
in  Gnetales,  219;  in  liverworts,  214;  in 
Lycopsida,  216;  in  phanerogams,  220; 
in  Pteropsida,  216 

Microtome,  452 

Middle  lamella,  3,  445 

Monocotyledons,  409;  amphivasal  bun- 
dles of,  196;  arrangement  of  bundles 
In,  192;  bundle  of,  193;  cambial  ac- 
tivity in,  195,  411;  characteristics  of, 
377;  geological  age  of,  409;  leaf  struc- 
ture in,  213,  410;  phylogeny  of,  412; 
stem  of,  192,  411;  root  of,  158,  410 

Mounting  of  sections,  459 

Mucilaginous  fibers,  33 

Mycorrhiza,  138 

N 

Nitrocellulose,  449 
Nothofagus,  parenchyma  of,  53 
Nucleus,  2 

O 

Oak,  wood  of,  14,  30,  55,  57 
Objects,  clamping  of,  455 
Oil  canals  of  Compositae,  433,  436 
Onopordon,  canals  of,  439 
Organs,  definitions  of,  136 
Origin  of  the  herbaceous  type:  in  HeJian- 

thus,  400,  402,  404;   in  Potentilla.,  187; 

in  Salvia,  189;    in  Solanum,  190;    in 

Urtica,  398 
Origin  of  the  woody  cylinder  in  Vitis,  391, 

395 
Osmunda  cinnamomea,  287,  289;  O.  clay- 

toniana,  289;  O.  regalis,  288 


Osmundaceae:    internal  phloem  in,  287; 

medulla  in,  286;  root  of,  20,  150 
Osmundites  skidegatensis,  286 


Palms,  anatomy  of,  196,  413 

Parenchyma.  37;  absence  of,  in  second- 
ary woods  of  the  Paleozoic,  40,  in  Pinus, 
41;  diffuse  in  conifers,  48,  in  dicotyle- 
dons, 50;  distribution  of,  in  Abies 
and  Tsuga,  47,  in  dicotyledons,  50,  52, 
in  primary  wood,  37,  38.  39;  origin 
of,  at  end  of  annual  ring,  42,  425; 
origin  of,  in  Alder,  54,  in  dicotyledons, 
54,  in  Liquidambar,  54,  in  Mesozoic,  40, 
in  Picea,  42,  425,  in  primary  wood,  37, 
39,  in  Prunus,  54,  in  secondary  wood, 
42,  54,  425;  origin  of,  from  tracheids, 
38,  42,  425;  relation  of,  to  annual  ring, 
45,  425,  to  climatic  evolution,  41,  425; 
terminal  in  dicotyledons,  52;  transition 
to,  from  short  tracheids,  43;  vasicentric 
in  dicotyledons,  52 

Parichni,  261 

Peach  stone,  4 

Pericycle  in  Pteris,  108 

Phloem:  central  ray  cells  in,  113,  115; 
companion  cells  of,  in  angiosperms, 
118;  in  Pinus,  no;  in  Pteris,  108;  in 
Tilia,  117;  marginal  cells  in,  113; 
parenchyma  in,  114;  rays  in,  114; 
sieve  tubes  in,  114;  in  Vitis,  122 

Phloeoterma,  10,  12 

Photographic  dark  room,  466 

Photographic  plates,  463,  467 

Photomicrography,  462 

Phylloglossum,  252 

Picea:  stem  of,  85 ;  terminal  parenchyma 
of,  336,  425;  wood  of,  42,  44,  425 

Picric  acid,  445 

Pileorhiza,  138 

Piliferous  layer,  146,  150,  158 

Pineae,  335 

Pinus:  absence  of  parenchyma  in,  336; 
aggregate  rays  in,  362;  leaf  of,  n,  330;  2 
needle  of,  n;  phloem  in,  no;  short 
shoots  of,  337;  structure  of  wood  in 
cone  of,  326;  tracheids  of,  26;  tyloses 
of,  106;  wood  of,  24,  326 

Pinus  and  Ginkgo:  microspores  of,  337; 
short  shoots  of,  337;  tracheids  of,  338 

Pinus  Strobus,  rays  in  phloem,  no 

Pit  membrane,  3,  6 

Pith,  origin  of,  in  the  Filicales,  283 


INDEX 


477 


Pits:  absence  of  tangential,  in  Cordaitales 
and  other  ancient  gymnosperms,  27; 
bordered,  6,  7;  condition  of,  in  heart- 
wood,  56;  half  bordered,  7 ;  radial,  27; 
simple,  3,  5,  6,  7;  tangential,  27 

Pitting,  distribution  of,  27,  428 

Pityoxylon,  structure  of,  349 

Plastid,  2 

Plates,  photographic,  463,  466 

Podocarpineae,  343 

Pollen  chamber,  226,  227 

Polydesmy  in  Cycadales  and  Gnetales,  364 

Porous  perforation  in  vessels,  origin  of,  101 

Potentilla,  origin  of  herbaceous  type  in, 
187 

Prepinus:  leaf  of,  207,  331;  relation  of, 
to  Cordaitales,  332 

Preservation  of  material,  444 

Primary  wood,  degeneracy  of,  169 

Principes,  anatomy  of,  196,  413 

Proangiosperms,  303 

Protocalamites,  268 

Protoplasm,  i 

Protostele,  162,  278;  collateral,  165;  con- 
centric, 162;  in  Filicales,  278 

Protoxylem,  18,  19,  20,  21 

Psilotum,  253 

Pteris:  bundle  of,  21,  108;  development 
of  stem  in,  280,  282;  phloem  of,  109; 
rhizome  of,  9,  10,  132;  vessels  of,  92 

Pteropsida,  244,  247;  geological  range  of , 
249;  megaphylly  of,  249 


Quercus:  fiber  tracheids  of,  31;  stem  of, 
179,  404;  tracheids  of,  31;  vessels  of, 
99;  wood  of,  15;  wood  rays  of,  76, 
178,  181. 

R 

Ranales,  characteristics  of,  386 

Rays:  aggregate,  78,  82;  compound,  78, 
82;  fusiform,  69,  74;  linear,  69; 
marginal  cells  of,  69;  medullary,  so 
called,  61;  origin  of,  in  Coniferales,  68, 
72,  in  Lepidodendron,  62,  63,  64;  rela- 
tion of,  to  resin  canals,  70;  tracheids 
of,  in  Lepidodendron,  62,  64;  types  of, 
in  Casuarina,  77;  uniseriate,  67,  69; 
width  of,  in  Cordaitales,  66,  in  Cyca- 
dales, 75,  in  Cycadofilicales,  65 

Recapitulation:  in  Coniferales,  235;  in 
first  annual  ring,  237;  in  oaks,  235;  in 
Phyllocladus ,  235 

Resin  canals,  relation  of,  to  resin  cells,  343 


Reversion,  72,  73,  74,  75,  241 

Rhizophore,  262 

Rhodotypus,  fibers  and  tracheids  of,  33 

Robinia  Pseudacacia,  mucilaginous  fibers 
of,  34 

Root:  absence  of  medulla  in,  173;  defini- 
tion of,  137;  development  of,  145; 
exarch  primary  structure  of,  145; 
exodermis  of,  158;  in  Coniferales,  145; 
in  dicotyledons,  151;  in  herbaceous 
dicotyledons,  156;  in  Lycopodium,  143; 
in  monocotyledons,  159;  in  Osmunda, 
150;  piliferous  layer  of,  158;  primary 
and  secondary  structure  of,  138;  pri- 
mary wood  of,  145,  150,  151,  157, 
159;  radial  organization  of,  143;  rays 
in,  147;  spiral  tracheids  in,  161; 
velamen  of,  159 

Root  cap  or  pileorhiza,  138 

Root  hairs,  146,  150,  158 


Sachs,  erroneous  views  of,  as  to  origin 
of  stem,  191,  406 

Safe  lights,  466 

Safranin,  458 

Salvia,  origin  of  herbaceous  type  in,  189 

Sapwood  or  alburnum,  55 

Scalariform  perforation  of  vessels,  origin 
of,  97 

Scolymus,  oil  canals  in  root  of,  441 

Scorzonera:  laticiferous  system  of,  435; 
oil  canals  of,  in  root,  441 

Secondary  wood  as  indicator  of  climatic 
conditions,  418 

Sectioning  of  material,  454 

Seed:  in  angiosperms,  chalazogamous, 
230;  in  Archigymnospermae,  226;  in 
Casuarina,  231;  in  dicotyledons,  232; 
in  monocotyledons,  231;  in  Pinus, 
233;  of  Coniferales,  229;  of  Cycadales, 
226;  of  Ginkgo,  228;  of  porogamous 
angiosperms,  230 

Seedlike  rnegasporangia,  223,  225 

Selaginella,  216,  225,  254 

Sequoia:  anatomy  of,  339;  cone  axis  of, 
340;  marginal  tracheids  of,  73,  341; 
parenchyma  of,  339;  tracheids  of,  25; 
wound  canals  of,  341 

Sharpening  of  knives,  453 

Shutter,  photomicrographic,  465 

Sieve  plates:  in  the  angiosperms,  120, 
123;  in  gymnosperms,  114;  in  her- 
baceous angiosperms,  125 


478 


THE  ANATOMY  OF  WOODY  PLANTS 


Sieve  tubes  and  vessels,  comparison  of, 
123 

Sigillaria,  primary  wood  of,  171,  259 

Sigillarians,  259 

Siphonostele,  163;  collateral,  165;  con- 
centric, 163;  in  Filicales,  278;  lacunae 
of,  164,  278;  reduction  of,  165 

Smilax,  root  of,  12,  138;  stem  of,  192 

Solatium,  origin  of  herbaceous  type  in, 
190 

Solidago,  resemblance  of  stem  of  certain 
species  to  that  of  the  oak,  405 

Sphenophyllaceae,  251,  264 

Sphenophyllum  insigne,  265 

Sporangia,  142;  in  Abietineae,  219;  in 
angiosperms,  220;  in  Coniferales,  219; 
in  Cycadales,  216;  in  Gink  go,  217;  in 
Gnetales,  219;  in  Polypodium,  216;  in 
Sdagindla,  .216 

Sporangia  and  trichomes,  130 

Staining  methods,  457 

Stangeria,  reproductive  axis  of,  299 

Starch:  in  phloem  of  Pinus,  in;  in 
phloem  of  Tilia,  118 

Stem:  definition  of,  140;  development 
of,  in  Pteris,  281;  herbaceous,  186,  387; 
organization  of,  in  Casuarina,  82,  in 
Cupressus,  80,  in  dicotyledons,  81,  in 
monocotyledons,  192,  in  Selaginella, 
254;  primary  tissues  in,  169;  second- 
ary tissues  in,  169;  structure  of,  in 
Cyathaceae,  282,  in  Marattiaceae,  282, 
in  Pteris,  281. 

Stephanospermum,  315 

Stigmaria,  262 

Stomata,  129 

Strops,  types  of,  454 

Substitute  fibers,  35 


Tangential  pitting  in  relation  to  climatic 
evolution,  424,  425 

Taraxacum,  laticiferous  system  of,  435 

Taxineae,  344;  parenchyma  in,  346;  re- 
productive structures  of,  345;  resin 
canals  in,  346;  tracheids  of,  345 

Taxodineae,  339,  marginal  tracheids  of, 
341;  wood  parenchyma  of,  342 

Taxoxylon,  structure  of,  349 

Teak,  septate  fibers  of,  35 

Terminal  parenchyma,  relation  of,  to 
climatic  evolution,  426 

Tissue  systems,  8,  10,  12,  13 


Tissues,  9;   epidermal,  10;   fibro vascular, 

10;   fundamental,  10 
Torus,  6,  68;  position  of,  in  heartwood,  56 
Trabeculae,  68,  250 
Tracheids,  25,  27,  29,  31;    marginal,  in 

Coniferales,  69,   73,  341;    reticulated, 

17;      ringed,     17;      scalariform,     17; 

spiral,  17;  spring,  24,  26;  summer,  24, 

26;  tylosis  of,  1 06;  types  of,  25,  26,  31 
Tragopogon,  oil  canals  in  root  of,  441 
Tree  nails,  34 
Trias,  woods  of,  421 
Tsuga:   parenchyma  of,  46;   resin  canals 

in,  346 

Tubiflorae,  oil  canals  of,  434 
Tyloses:  in  fibers  of  Liquidambar,  106;  in 

resin    canals    of    Coniferales,    56;     in 

vessels  of  dicotyledons,  57 

U 

Urtica,  organization  of  the  stem  in,  397 


Vacuole,  i 

Vasicentric  parenchyma,  51 

Velamen,  160 

Vessels:  absence  of,  in  certain  angio- 
sperms, 373,  and  sieve  tubes,  123;  in 
Ephedra,  94;  in  Gnetum,  95;  in  Pteris, 
92;  inWelwitschia,  95;  lateral  walls  of, 
103;  scalariform,  103;  terminal  walls 
°f>  93>  95  >  tylosis  of,  104;  types  of, 
94,  95,  100,  101;  with  pitted  perfora- 
tion, 94;  with  porous  perforation,  96, 
99;  with  scalariform  perforation,  96 

Vine  type,  origin  of,  184,  390,  396,  397 

Vitis,  organization  of  stem  in,  185,  391; 
thick  and  thin  annual  stem  of,  397 

Voltzia,  355 

W 

Welwitschia,  366;  leaf  of,  368;  stem  of, 
366;  vessels  of,  366 

Wood:  cryptogamic,  222;  holders  for, 
455;  primary,  16,  18,  20,  22,  167; 
tangential  pits  of,  425 

Wood  parenchyma:  origin  of,  in 
Abietineae,  41,  43;  in  Araucariineae, 
320,  322;  in  Podocarpineae,  344;  in 
Taxineae,  346;  in  Taxodineae,  339,  342 

Woody  dicotyledons,  379 

Woody  tissues,  softening  of,  447 


Xylem:    endarch,   21,   167;    exarch,   19, 
167;  mesarch,  20,  167 


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